US2378056A

US2378056A - Resistance unit and element thereof

Info

Publication number: US2378056A
Application number: US473351A
Authority: US
Inventors: David C Wright
Original assignee: Electric Controller and Manufacturing Co LLC
Current assignee: Electric Controller and Manufacturing Co LLC
Priority date: 1943-01-23
Filing date: 1943-01-23
Publication date: 1945-06-12
Anticipated expiration: 1962-06-12

Description

June 12, 1945. c. WRIGHT RESISTANCE mm: AND ELEMENT manor" Filed Jan. 23, 194::

4 Sheets-Sheet 1 INVENTOR. DAVID C. WRIGHT June 12, 1945. c wRlGHT 2,378,056

RESISTANCE UNIT AND ELEMENT THEREOF Filed Jan. 25, 1943' 4 Sheets-Sheet 2 V i'/ if R? g 70 J\/ &

INVENTOR.

DAVID C. WRIGHT BY %%)Zmm/, 4W.

June 12; 1945. D c, wRlGHT 2,378,056

RESISTANCE UNIT AND ELEMENT THEREOF Filed Jan. 25, 1945 4 Sheets-Sheet A INV ENTOR. DAV! 0,. C. WRIQ H BY June 12, 1945.- D, WRl HT 2,378,056

RESISTANCE UNIT AND ELEMENT 1HEREOF Filed Jan. 23, 1943 4 Sheets-Sheet 4 INVENTOR. 9 DAvlo J/ R GH Patented June 12, 1945 RESISTANCE UNIT AND ELEMENT THEREOF David C. Wright, Euclid, Ohio, assignor to The Electric Controller 3; Manufacturing Company, Cleveland. Ohio, a corporation of Ohio Application January 23, 1943, Serial No. 473,351

24 Claims.

This invention relates to electrical resistance elements and electrical resistance units containing the same, and more particularly to resistance elements having an improved relation between supporting lugs and an intermediate resistance strip and to resistance units comprising an assemblage of a plurality of such resistance elements and suitable for various purposes such as, for example, the control of electric motors.

Heretofore, electrical resistance units have been constructed by tightly clamping together in parallel spaced relationship either a plurality of cast metal resistance elements or a plurality of sheet metal resistance elements between end frames by means of suitable insulated bolts which pass through perforations in lugs formed at the ends of the elements. The resistance elements generally are in the form of grids in order to obtain a maximum length of current conducting path for unit volume of the assembled resistance unit.

In the case of prior cast resistance grids, the end lugs generally comprise integral annular bosses which extend outwardly an equal distance from both sides of the plane of the grid at each end thereof and serve to space apart the intermediate portions of adjacent grids when the grids are supported on the insulated bolts which pass through the openings in the bosses. Usually, when an adjacent pair of the end lugs of such cast grids are to be in conducting relationship, a thin soft metal washer, such as a copper washer, is slipped over the clamping bolt between the opposing flat surfaces of the bosses of the adjacent end lugs. Consequently.-when the cast grids are clamped tightly together between the end frames, satisfactory current conducting paths of low and substantially uniform resistance are made between the lugs of adjacent grids. As to adjacent end lugs which must be insulated from each other, a thin washer of insulating material is slipped over the clamping bolt between the opposing flat surfaces of the bosses. Because of the inherent short distance between the opposing surfaces of the bosses of adjacent grids, the insulating washers must be made from a material having good insulating properties and are therefore generally formed from mica. and have a diameter considerably larger than that of the bosses in order to increase the creepage distance.

Prior resistance grids formed of sheet metal have been flat throughout their length, and therefore relatively thick washers or tubular spacers are required between the end lugs of adjacent grids in order to space the grids apart when assembled in a resistance unit. These spacers are generally formed of metal and serve as conductors between adjacent end lugs where conduction is required, and are spaced from the adjacent end lugs that are to be in non-conducting relationship by thin insulating washers. As in the case of cast grids with annular bosses, these insulating washers must be of good insulating material and of large diameter since the distance between the grids and the metal spacers is extremely short.

For greatest continuous uniformity in the resistance of a resistance unit it is essential that the number of metal-to-metal joints in the circuit through the unit be as few as possible. In the prior types of resistance units, the use of copper washers or metal spacers necessitates two metal-to-metal joints between adjacent grids whereas but one is the optimum number, and the extra pieces required increases the cost of the unit both as to material expense and assembly time. An object of this invention is to provide a resistance unit comprising a plurality of resistance elements supported in substantially parallel spaced relationship with respect to each other in which the number of metal-to-metal joints in the circuit through the unit and the number of pieces used in the assembly of the unit are reduced substantially to a minimum.

The insulators used between adjacent end lugs of prior cast grids are much shorter than the distance between the intermediate portions of adjacent grids, and, even if the metal spacers between insulated end lugs of prior sheet metal grids are replaced by tubular insulators, the insulators in the case of the prior sheet metal grids can be no longer than the distance between the intermediate portions of adjacent grids. In order to have a compact resistance unit the distance between the intermediate portions of adjacent grids should be as short as possible consistent with good thermal characteristics, whereas for simplicity in providing adequate insulation the distance between adjacent end lugs that are to be insulated from each other should be as long as possible. It is a further object of this invention to provide a. resistance unit comprising a plurality of parallel spaced resistance elements in which the insulating distance between the lugs of adjacent resistance elements is longer than the distance between the intermediate portions of the adjacent elements.

Heretofore, resistance grids of either cast or sheet metal have been held in current conducting relationshi by tightly clamping them between metal end frames. Such construction in the case of sheet metal grids which have a relatively large thermal coefficient of expansion has led to the use of special floating mountings between the grid supports and the end frames in order to minimize as much as possible the eflect of strains due to heating. Although such floating mountings have been of some value, certain applications of resistance units require that a part only of the unit be raised to a high temperature while the remainder of the unit remains at substantially normal temperature. This uneven heating causes thermal strains which cannot be compensated for by permitting relative movement between the supporting bolts and the end frames. Another object of this invention is to provide a resistance unit comprising a plurality of arallel spaced sheet metal resistance elements in which the necessity for special mounting construction is eliminated and the principal difilculties due to thermal expansion are avoided.

In accordance with one of the forms which this invention may take, the end lugs of a resistance element or grid are offset in opposite directions from, and lie in planes respectively which are parallel to, the plane of the intermediate portion of the grid. The planes of the end lugs are spaced from the plane of the intermediate portion by a distance equal to one-half of the required spacing between the intermediate portions of adjacent grids. The grids are mounted on a pair of supporting bolts between a pair of end frames but, in the case of sheet metal grids, are not necessarily clamped tightly together. If sheet metal grids are used, a current conducting path of low and uniform resistance is obtained between loosely clamped grids by spot welding together the end lugs of the assembled grids which are offset toward each other. By proper selection or" the relative direction of offset of the end lugs of the grids, tubular insulators which are longer than the distance between the intermediate portions of adjacent grids, may be used between end lugs which are to be insulated from each other, and, at the same time and in consequence of the same selection, the end lugs which are to be in conducting relation with each other are in face to face contact thus obviatin the necessity of using separate metal spacers, providing conduction from grid to grid through a single metal-to-metal joint, and permitting simplified welding of the grids. Besides eliminating the necessity of tightly clamping the grids together, welded connections between the grids provide current conducting paths between grids which have uniform resistance over long periods of time. In prior non-welded resistance units oxidation of the contacting surfaces of the adjacent lugs resulted in resistance variations.

Another object of this invention is to provide new and improved resistance elements which. when assembled to form a resistance unit, result in a resistance unit of improved design.

A correlative object is to provide a resistance element of new and improved configuration which permits simplified assembly of an electrical resistance unit containing the same.

Another object is to provide a new and improved electrical resistance unit comprising resistance elements of novel design.

Another object is to provide mutually cooperating resistance elements having such a relationship between supporting lugs and an intermediate resistive strip that said elements may be assembled into a compact resistance unit without the use of metal spacers between adjacent elements and which unit does not require the use of resistance material of high dielectric strength between adjacent elements.

A more specific object is to provide complementary electrical resistance grid of uniform thickness throughout which have ofl'set end portions and a resistance unit comprising a plurality of such grids.

A further specific object is to provide an electrical resistance unit comprising a plurality of resistance elements held in conducting relationship by welded connections.

Other objects and advantages of this invention will become apparent from the following specification wherein reference is made to the accompanying drawings, in which:

Fig. 1 is a perspective view of one of the resistance elements of this invention;

Fig, 2 is a perspective view of another of the resistance elements of this invention and which is complementary to the element of Fig. 1;

Fig. 3 is a bottom perspective view illustrating a resistance unit embodying the resistance elements of Figs. 1 and 2;

Fig. 4 is a sectional elevational view of a resistance unit embodying the resistance elements of Figs. 1 and 2 and taken generally as indicated at 4-4 in Fig. 5;

Fig. 5 is a sectional view taken generally as indicated at 5-5 in Fig. 4, part of the insulation and one of the supporting bOlts being broken away and the intermediate supporting means being omitted;

Fig. 6 is a side elevational view from the left of Fig. 4 with the intermediate supporting means being omitted;

Fig. 7 is a perspective view of a modified form of resistance element in accordance with this invention;

. Fig, 8 is a bottom plan view illustrating a resistance unit embodying the resistance elements of Fig. 7;

Fig. 9 is a bottom plan view illustrating a multiple resistance unit embodying the resistance elements of this invention; and

Fig. 10 is a perspective view of a part of the resistance unit of Fig. 9.

Referring to Figs. 1 and 4., one of the individual resistance elements of this invention, indicated by the reference numeral I0, is an integral piece of sheet metal of suitable electrical characteristics in the form of a grid having a pair of oppositely offset end lugs l2 and H of identical configuration with perforations l6 and I8 therein. respectively. For the purpose of facilitating welding between adjacent grids,

rectangular welding tongues

20 and 22 extend downwardly from the outer edges of the lugs l2 and I4, respectively. The lugs l2 and H are connected by angularly disposed

strip portions

24 and 25, respectively, to opposite ends of an intermediate fiat sinuous strip 26 having a suitably arranged co-planar series of convolutions 28 forming a circuitous current path between the lugs l2 and II. As viewed from the left in Fig, l, the lug I4 is offset to the left of and lies in a plane parallel to the plane of the strip 26 by virtue of the angularly disposed strip portion 25, and the lug I2 is offset to the right of and lies in a plane parallel to the plane of the strip 26 by virtue of the angularly disposed strip portion 24.

Another of the individual resistance elements or grids of this invention, indicated by the reference numeral 39 and shown best in Fig. 2, is the same as the grid ill in all respects except as to the direction of offset of integral end lugs 92 and 94 which correspond to the end lugs l2 and I4, respectively, of the grid Ill. The

lugs

32 and 94 are offset oppositely with respect to each other, have the perforations l9 and I9 therein. respectively, and include downwardly extending rectangular welding tongues 45 and 39, respectively. The

lugs

32 and 34 are connected by angularly disposed strip portions 41 and 99. respectively, to opposite ends of an intermediate flat sinuous strip 4 identical with the strip 29 of the grid ll. As viewed from the left in Fig. 2, the lug 94 is offset to the right of and lies in a plane parallel to the plane of the strip 49 by virtue of the angularly disposed strip portion 38, and the lug I2 is offset to the left of and lies in a plane parallel to the nlane of the strip 49 by virtue of the angularly disposed strip portion 91.

As will be described in detail hereinafter. the grids l and all are comp ementary to each other when assembled in a. resistance unit. and thus the grid I9 may be considered a left hand grid and the grid 39 may be considered a right hand grid.

The grids i9 and may be formed directly from sheet metal to the general shape shown by a stamping operation or may be formed by edgewise bending of a long straight strip. Some of the advantages of this invention also may he 1 obtained by making the grids of cast metal. The sinuous strips 29 and may be of any suitable configuration, the convolutions 29 as shown be ng merelv illustrative of a preferred embodiment.

Referring now to Figs. 3. 4. 5 and 6. a series connected electrical resistance unit 4| of the present invention comprises a plurality of the complementary grids l0 and 30 supported between suitable end frames 42 and 44. A pair of elonated supporting bolts and 41 are inserted through horizontally spaced p rforations near the top corners of one of the end frames such as 41. Each of the

bolts

48 and 41 is preferably insulated from the frame 42 by an outer fiber washer 49 having a centrally disposed annular boss 49 (Fig. 5) which extends throu h the perforation in the a terminal 99 formed from a flat strip of conducting metal and having a perforation 62 near one end for the attachment of an electrical conductor (not shown) and a u-shaped slot 64 (Fig.

'4) at the other end which is adapted to fit over either of the

tubes

56 and 51 is mounted on the tube 51 in face to face contact with the lug l4 of the first of the grids Hi. The terminal BI! is provided with a rectangular welding tongue 66 of the same general configuration as the

welding tongues

29, 22, 95 and 16 which extends downwardly from the bottom edge of the terminal 90 near the end having the slot 84 in alignment with the welding tongue 22 of the first one of the grids l0.

After the first one of the left hand grids l0 and the first one of the terminals 80 are in place, a tubular insulator 10 similar to but longer than the

insulators

58 and 59 is slipped over the tube 51. and then the first one of the right hand grids 30 is mounted on the

bolts

46 and 41 by slippin the perforation IS in the lug 32 over the tube 56 and by slipping the perforation IS in the lug 34 over the tube 51. The length of the insulator 19 is so selected that when the lug 32 abuts against I the lug l2 and the lug 34 touches the end of end frame 42 intoa complementary annular shouldered recess around a central opening of an inner fiber washer 50. and is retained in position by a check nut 5| and a clamping nut 52, metal washers 54 serving the usual function as shown. Suitable insulating

tubes

56 and 51 are slipped over the

bolts

48 and 41, respectively, and

suitable i sulators which may be in the form of stacked mica washers are then slipped over the

tubes

55 and 51 respectively and engage the innermost washers 54 respectively to provide a good insulating barrier between the end frame 42 and the resistance grids l0 and 30.

A suitable tubular insulator 58 which may be of durable ceramic-like material is slipped over the tube 51 against the insulator 55. and a s milar but longer insulator 59 is slipped over the tube 56 against the other insulator 55. The first of the left hand grids I0 is then mounted or. the bolts 45 and 41 by slippina the perforation I6 in the lug l2 over the tube 56 and by slipping the perforation l9 in the lug I4 over the tube 51. The relative lengths of the

insulators

58 and 59 are so selected that when the lug |2 engages the end of the insulator 59 and the lug l4 engages the end of the insulator 59 the first of the grids I0 is substantially parallel with the end frame 42,

After the first one of the grids i0 is in place,

the insulator 10, the first of the grids 30 is substantially parallel with the end frame 42. After an additional insulator 10 is slipped over the tube 55, the second one of left hand grids I0 is slipped over the

tubes

56 and 51 so that the offset lug l4 fits ti htly against the offset lug 34 of the first of the grids 30 and the offset lug I2 engages the end of the insulator 10 on the tube 56.

In like fashion a suitable number of additional grids l0 and 30 are slipped alternately over the

tubes

59 and 51 with the oppositely offset lugs alternately in face to face contact or spaced apart by one of the insulators 10. Thus the grids I0 and 30 are arranged in parallel spaced relationship with respect to each other on the

tubes

56 and 51 with alternate opposite lugs of adjacent grids in face to face contact.

Additional terminals 60 may be placed over

th tubes

56 or 51 between adjacent pairs of the lugs i2 and 32 or M and 94 or in contact with any one of the end lugs at suitable intervals depending upon the amount of resistance required between the respective terminals 60. Compensation may be made in the length of any of the insulators 10. if necessary. to allow for the s ace occupied by the terminals 60 so that all of the grids Ill and 30 in the unit 4| are in substantially parallel spaced relationship with respe t to each other. The welding tongues 20. 22, 35. 36 and 66 all have the same general configuration and are in face to face contact when their associated end lugs or the terminal 50, as the case might be, are in face to face contact.

After the entire stack of resistance grids Hi and 30 has been assembled, additional insulators 55. 58. and 59 are slipped over the

tubes

56 and 51 and

additional washers

50 and 54 and nuts 5| are grouped on the

bolts

46 and 41 which are then inserted in horizontally s aced perforations in the end frame 44 and furnished with

additional washers

48 and 54 and clamping nuts 52. Thus. t e grids l0 and 30 are spaced and insulated fr m the frame 44 and the

bolts

46 and 41 are supported by and insulated from the frame 44 by the same typ of spacing. insulating. and clamping arrangement used f r the end frame 42. The length of the

resistors

58 and 59 at opposite ends of the resistance unit 4| may be selected so as to provide uniform spacing distances between the end frames 42 and 44 of resistance units having the same number of grids regardless of the thickness of the metal forming the individual grids. The fact that the

bolts

40 and 41 are insulated from the end frames 42 and 44 gives increased protection from grounds through the end frames, and the wall thickness of the

tubes

56 and 51 therefore may be less than that required if the bolts 45 and 41 were allowed to touch the end frames.

In each instance where two or more of the

welding tongues

20, 22, 85, 30, or 65 are in face to face contact, a pair of spot welds I2 is made through the adjacent welding tongues to form the stack of grids tightly into an integral resistance structure. The fact that all of the joints between the grids I and 30 and the terminals 00 are welded makes it unnecessary to clamp the grids and the terminals tightly together between the end frames. Space for thermal expansion therefore may be provided by adjustment of the nuts and 52 so that all deleterious thermal strains are avoided.

As can be clearly seen in the drawings, the assembled resistance unit 4| provides a series current conducting path which is alternately through the parallel aced grids l0 and 30. The circuit through the resistance unit 4I illustrated extends from the terminal 60 closest to the end frame 42 through one pair of the spot welds I2 to one of the grids I0, another pair of the spot welds I2, one of the grids 30, and so on through the welded grids I0 and 30 alternately and any intermediate terminals 50, until the final terminal 50 closest to the end frame 44 i reached.

For some applications of theresistance unit 4| it might be desirable to have the terminals 60 removable for easy adjustment, in which case the welding tongues 06 may be omitted. The grids I0 and 30 can then be welded together as before and the terminals 60 slid against the lugs I2, I4, 32. or 34 and held in circuit making engagement by the nuts 52, the check nuts 5| not being necessary if direct clamping action is to be used. It is also obvious that suitable terminals adapted to be clamped to the grids I0 and 30 could be used instead of the terminals 50.

As most clearly shown in Figs. 5 and 6, the insulators I0 are longer than the distance between the intermediate portions of the parallel spaced grids I0 and 30 due to the cooperative relation of the offset end lugs. The extra length of the insulators I0 permits them to have a smaller diameter than otherwise would be necessary and to be made of a lighter weight material of lower dielectric strength than heretofore possible. Furthermore, there is no need of using mica washers between adjacent grids. The cooperative action between the offset lugs of the complementary grids I0 and 00 also reduces to a minimum the number of metal to metal joints through the resistance unit and the number of separate pieces required to assemble the complete resistance unit.

In resistance units made up of grids formed from sheet metal of extremely light gauge, supplementary intermediate supporting means may be required. Such additional support for light gauge grids may be provided in a well known manner as indicated in Fig. 4 which shows a pair of elongated bolts I4 and I6 surrounded by insulating

tubes

18 and 80, respectively. The tubes I8 and 80 pass between and contact the convolutions 20 of the

sinuous strips

25 and 40 and through spaced perforations in each of the end frames 42 and 44 to which they may be suitably fastened. Insulating spacers 02 are slipped over the tubes I0 and 00 respectively between each of the grids I0 and 00. The diameter of the insulators 02 is larger than the distance between the convolutions 20, and the grids I0 and 00 of thin gauge are thus held from sagging or vibrating into contact with each other.

The modified form of resistance grid of Fig. 7 is suitable for assembling into a resistance unit in which pairs of grids are connected in parallel electrically and the pairs so connected are connected in series with each other as shown in Fig. 8. The grid 04 or Fig. '7 is preferably the same as the grids I0 and 00 in all respects except as to the relative direction of the oflset of integral end lug 00 and 00 which correspond to the end lugs I2 and I4, respectively, of the grid I0. The lugs 00 and 00 are both offset in the same direction from the intermediate portion of the grid 04 instead of in opposite directions as in the case of the grids I0 and 00, but have the perforations I6 and I8 therein, respectively, and include downwardly extending

rectangular welding tongues

90 and 92, respectively. The lugs 86 and 08 are connected by angularly disposed strip portions 04 and 95, respectively, to opposite ends of an intermediate flat sinuous strip 90 which is preferably identical with the strip 20 of the grid I0.

Referring to Fig. 8, a parallel connected resistance unit I00 of this invention comprises a plurality of the grids 04 supported in parallel spaced relationship on the elongated supporting

bolts

46 and 41 between a pair of end frames (not shown) such as the frames 42 and 44 of the previous embodiment. The grids 04 are arranged in pairs with the offset lug of each grid of the pair facing the offset lug 80 of the other grid of the pair. Each of the pairs thus comprises two of the grids 04 connected in parallel electrically, and the pairs are connected in series with each other in a manner to be described. One of the terminals 00 is inserted over the tube 51 on the bolt 41 between the lugs 00 and B0 of a first pair IOI of the grids 04 and the mutually engaging welding tongues 00, 02, and 66 may be spot welded together as before. The first pair III of the grids 84 may be spaced from the end frame by a pair of the mica insulators 55 and a pair of the ceramic insulators 10 of suitable length, insulators 10 also being used to space apart the succeeding pairs of grids from each other as shown.

At the end of the first pair I0| of the grids 04 opposite from the terminal 60, a leg I04 of a U- shaped jumper I05 is inserted between the

lugs

86 and 80. Another leg I08 of the jumper I00 is spaced from the leg I04 by a bight portion H0 and is inserted between the lugs 06 and 00 of a second pair II2 of the grids 84. Each of the legs I04 and I 00 of the jumper I06 is similar in general shape to the terminal 50 and includes a downwardly extending welding tongue H0, the bottom edge of which is shown in Fig. 8. Additional jumpers I05 serve to interconnect alternate opposite ends of additional pairs of the grids 04 throughout the complete resistance unit I00 so that the electrical circuit through the unit passes through a series connection of a plurality of parallel connected pairs of grids 84. Each of the bight portions N0 of the jumpers I05 may be provided with an opening H4 to facilitate attachment of additional terminal conductors.

The resistance unit I00 may be welded into a unitary structure by spot welding between the

welding tongues

88, 82 and H3 wherever two or more of the welding tongues are in face to face contact and an additional terminal 60 may be welded to a last pair of grids at the other end of the resistance unit.

The resistance unit II4 of Fig. 9 is adapted to be assembled as a part of a heavy duty, face plate, manual controller for an electric motor and illustrates one way in which the grids I0, 38, and 84 of this invention may be slightly modified and then combined to form a multiple resistance unit. A plurality of elongated bolts H6, H8, I20, and I22 are surrounded by insulating tubes I24, I26, I28, and I38, respectively. The tubes I24 to I38 are adapted to pass through the perforations in the lugs of the grids III, 30, and 84 and the bolts II6 to I22 may be supported near their opposite ends by a suitable end supporting means (not shown) which may be a framework adapted to fit within the housing of a manual controller. The insulators 56 surround the tubes I24 to I30, respectively, near one end thereof to serve as insulation between the grids and the end supporting means of the unit H4, and the insulators III surround the tubes I26, I28, and I38, respectively, in engagement with the respective insulators 55.

The grids I8, 30, and 84 are arranged in alternate complementary groups I33 and I34 which extend transversely of the bolts III; to I22 in parallel spaced relation with each other so that the groups I33 and I34 have alternate opposite end lugs in face to face contact which may be welded together. The group I33 comprises one of the grids II) and two of the grids 84 connected in end to end relationship, and the group I34 comprises one of the grids 38 and two of the grids 84 connected in end to end relationship. The resistance groups I33 and I34 may be considered broadly as resistance elements having oppositely offset end lugs.

The lug I2 of the grid III of the first group I33 surrounds the tube I24 and abuts against one of the insulators 55. The lug 86 of the intermediate one of the grids 84 surrounds the tube I26 and abuts against one of the insulators Ill and is in face to face contact with the lug I4 of the grid I which also surrounds the tube I26. When the lug I2 of the grid I0 is against the insulator 55 on the tube I24, the lugs 86 and I4 are in contact with each other, the lug 86 being between the insulator III on the tube I26 and the lug I4. The welding tongue 30 of the grid 84 is out 01f at I34 as shown more clearly in Fig. in order to'provide clearance for the angularly disposed portion of the grid III. The lug 88 of the intermediate grid 84 surrounds the tube I28 in engagement with one of the insulators I0 and the lug 86 of the end one of the grids 84 of the group I33 surrounds the tube I28 and is in face to face contact with the lug 88 of the intermediate grid 84. The lug 88 of the end grid 84 surrounds the tube I38 in engagement with one of the insulatorsi"I0;"Welded connections I36 may now be made between the

welding tongues

22 and 88 and the portion of the end lugs 86 and I4 in face to face contact therewith, respectively, as indicated generally in Fig. 10. Similarly the end lug 88 of the intermediate grid 84 may be welded to the lug 86 of the end grid 84, the welding tongue 82 being shortened to accommodate the angularly disposed portion 96 of the intermediate grid 84.

One of the terminals 60 is mounted over the tube I 24 adjacent to the lug I2 of the first one of the grids III and its welding tongue 66 is preferably welded to the welding tongue 28 of the first one of the grids I8. Additional insulators III of suitable length respectively surround the tubes I24, I26 and I28 in contact with the lugs of the first group I33 of grids. The lug 34 of the grid 38 of the first group I34 of grids surrounds the tube I38 and is in face to face contact with the lug 88 of the end grid 84 of the group I33 and the

welding tongues

36 and 82 are welded together. Each of the grids of the group I34 are welded together as the corresponding grids of the group I33, the welding tongues of the lugs supported by the bolts H8 and I28 beingshortened where necessary. Welded to the lug 88 of the end grid 84 of the first group I34 of grids is the lug I2 of the grid ll of the second group I33, and the resistance unit is completed as thus indicated.

I claim:

1. An electrical resistance element adapted to be assembled in a resistance unit including a plurality of said resistance elements arranged in parallel spaced relationship and comprising a pair of metal mounting lugs connected by an intermediate resistive portion, substantially all of said intermediate portion lying substantially in the same plane, both of said mounting lugs extending outwardly in opposite directions from opposite ends of said intermediate portion and being offset in opposite directions respectively from the plane of said intermediate portion and lying in planes respectively which are substantially parallel to the plane of said intermediate portion.

2. An electrical resistance element adapted to be assembled in a resistance unit including a plurality of said resistance elements arranged in parallel spaced relationship and comprising a piece of metal having a Pair of mounting lugs connected by an intermediate strip portion, substantially all of said strip portion lying substantially in the same plane, the mounting lugs and strip portion being of substantially the same thickness, and at least one of said lugs being offset from the plane of and in non-overlapping relation with respect to said intermediate strip portion and lying in a plane substantially parallel to the plane of said intermediate strip portion.

3. An electrical resistance element adapted to be assembled in a. resistance unit including a plurality of said resistance elements arranged in parallel spaced relationship and comprising a piece of metal having a pair of mounting lugs connected by an intermediate strip portion, substantially all of said strip portion lying substantially in the same plane, the mounting lugs and strip portion being of substantially the same thickness, and both of said lugs extending outwardly in opposite directions from opposite ends of said intermediate strip portion and being offset from the plane of said intermediate strip portion.

4. An electrical resistance element adapted to,

be assembled in a resistance unit including a plurality of said resistance elements arranged in parallel spaced relationship and comprising a piece of metal having a pair of mounting lugs connected by an intermediate strip portion, substantially all of said strip portion lying substantially in the same plane, the mounting lugs and strip portion being of substantially the same thickness, and both of said lugs being offset from the plane of and in non-overlapping relation with respect to said intermediate strip portion and lying in planes respectively which are substantially parallel to the plane of said strip portion.

5. An electrical resistance element adapted to be assembled in a resistance unit including a plurality of said resistance elements arranged in parallel spaced relationship and comprising a piece of metal having a pair of mounting lugs connected by an intermediate strip portion, the mounting lugs and strip portion being of substantially the same thickness, and both of said lugs being offset in the same direction from the plane of and in non-overlapping relation with respect to said strip portion.

6. An electrical resistance grid comprising a pair of mounting lugs connected by an intermediate sinuous flat resistive strip forming a circuitous current conducting path, both of said lugs being offset in opposite directions respectively from the plane of and in non-overlapping relation with respect to said intermediate strip and lying in planes respectively which are substantially parallel to the plane of said intermediate strip.

7. A resistance grid comprising an integral piece of sheet metal having a pair of flat supporting lugs connected by an intermediate flat strip forming a circuitous current conducting path between said lugs, said strip having angularly disposed portions adjacent each of said lugs causing said lugs to be offset from the plane of and in non-overlapping relation with respect to said intermediate strip and in planes repectively which are substantially parallel to the plane of said intermediate strip.

8. A resistance element in accordance with claim 1 characterized in that at least one of said lugs includes means for supporting the element and has a projecting portion coplanular with the lug to facilitate welding between adjacent elements when said elements are assembled in parallel spaced relation.

9. An electrical resistance unit comprising a plurality of separate resistance elements each having a resistive portion intermediate a pair of lugs, substantially all of said resistive portion lying substantially in the same plane, the lugs and resistive portion of each element being of substantially the same thickness, at least one lug of each of said elements being offset from the plane of and in non-overlapping relation with respect to the resistive portion and lying in a plane which is substantially parallel to the plane of said intermediate portion, and supporting means for said elements, said elments being supported by said supporting means in substantially parallel spaced relationship with respect to each other and with each of said offset lugs extending toward an adjacent element.

10. An electrical resistance unit comprising a supporting means, a plurality of resistance elements each having a pair of lugs connected by an intermediate portion, substantially all of said intermediate portion lying substantially in the same plane, at least one lug of each of said elements being offset from the plane of and in non-overlapping relation with respect to said intermediate portion of the element and lying in a plane which is substantially parallel to the plane of said intermediate portion, and said elements being supported by said supporting means in substantially parallel spaced relationship with respect to each other with each of said offset lugs extending toward a lug of an adjacent element and in electrical contact therewith.

11. An electrical resistance unit comprising a supporting means, a plurality of resistance elements each having a pair of lugs connected by an intermediate portion, both of said lugs of each of said elements being offset from the plane of the intermediate portion of the element, and said elements being supported by said supporting means in substantially parallel spaced relationship with respect to each other with each of said offset lugs extending toward a lug of an adjacent element and in electrical contact therewith.

12. An electrical resistance unit comprising a supporting means, a plurality of resistance elements each having a pair of lugs connected by an intermediate portion, both of said lugs of each of said elements being offset in opposite directions respectively from the plane of the intermediate portion of the element, and said elements being supported by said supporting means in substantially parallel spaced relationship with respect to each other with each of said offset lugs extending toward a lug of an adjacent element and in electrical contact therewith.

13. An electrical resistance unit comprising a plurality of resistance elements each having a pair of lugs interconnected by an intermediate resistive portion, at least one of the lugs of each of said elements being offset from the plane of the intermediate portion, and supporting means for said elements, said elements being arranged in substantially parallel spaced relationship with respect to each other on said supporting means with alternate opposite lugs of adjacent elements operatively in face to face contact with each other.

14. An electrical resistance unit comprising a supporting structure, a plurality of resistance grids supported in parallel spaced relationship by said supporting structure, each of said grids having an offset end lug extending away from the remainder of the grid and lying in a plane parallel to the plane of and in a non-overlapping relation with respect to the grid and in face to face contact with an offset end lug of an adjacent grid, each of said grids having a second offset end lug extending away from the remainder of the grid and lying in a plane parallel to the plane of the grid and on the opposite side thereof from the plane of said first offset end lug, whereby, when one pair of lugs of adjacent grids are in face to face contact, the other pair of lugs of said adjacent grids are spaced apart by a distance greater than the distance between the planes of the adjacent grids.

15. An electrical resistance unit comprising a plurality of resistance elements each having a pair of oppositely offset lugs interconnected by an intermediate fiat resistive portion, each of said lugs lying in planes respectively which are parallel to the plane of the resistiv portions of the element and being on the same side of a longitudinal plane through the longitudinal axis of said element normal to the plane of said resistive portion, whereby said elements are unsymmetrical about said longitudinal axis, a first group of elements having the relativendirec tion of offset of the lugswithrespect to said plane'Bf'tnrx-F sistive portion being reversed from the relative direction of offset of the lugs of a second group of said elements with respect to said plane of the resistive portion, supporting means for said elements, individual elements of said groups being arranged alternately in parallel spaced relationship with respect to each other on said supporting means, whereby one adjacent pair of lugs of a pair of adjacent elements extend toward each other while the other adjacent pair of lugs of said pair of elements extendaway from each other. V

16. An electrical resistance unit comprising a plurality of resistance elements each having a pair of end lugs offset in the same direction from and in non-overlapping relation with respect to an intermediate fiat sinuous resistive portion, supporting means for said elements, pairs of said elements supported by said supporting means in parallel spaced relationship with adjacent lugs of the elements of each pair in electrical contact with each other, and electrical conductors between alternate opposite ends of adjacent pairs of elements.

17. An electrical resistance unit comprising a plurality of groups of resistance elements, each group including a plurality of resistance elements connected serially in end to end relation with respect to each other, each of said elements having a pair of offset end lugs, a plurality of insulated supporting rods extending through openings in said lugs for supporting said groups of elements in parallel spaced relation with respect to each other, the offset lugs at alternate opposite ends .of the groups extending toward and in face to face contact with each other, and alternate opposite offset lugs 01' the other ends of said groups extending away from each other.

18. An electrical resistance unit in accordance with claim characterized in that an elongated supporting bolt passes through said lugs and in that welded connections are provided between the lugs which are. in electrical contact with each other.

19. An electrical resistance unit comprising a supporting structure, a plurality of resistance elements each having a pair of mounting lugs extending outwardly in opposite directions from opposite ends of a mono-planular intermediate portion and offset in opposite directions respectively from the plane of said portion, said lugs lying 'in planes respectively which are parallel to the plane of the intermediate portion, and means associated with said lugs and supporting said elements on said supporting structure in parallel spaced relationship with alternate opposite pairs of lugs of adjacent elements in face to face contact.

20. An electrical resistance unit comprising a supporting structure and a plurality of pairs of complementary resistance elements, each of said elements having a pair of mounting lugs extending outwardly in opposite directions from opposite ends of a mono-planular intermediate portion and offset in opposite directions respectively from the plane of said portion, said lugs lying in planes respectively which are parallel to the plane of the intermediate portion, and means associated with said lugs and supporting said complementary elements alternately with respect to each other on said supporting structure in parallel spaced relationship and with alternate opposite pairs of lugs of adjacent complementary elements in face to face contact.

21. An electrical resistance unit comprising a supporting structure, a plurality of resistance elemeats each having a pair of mounting lugs offset in planes parallel to the plane of the remainder of the element and in non-overlapping relationship with respect to the remainder of the element, means cooperating with said lugs for supporting said elements on said structure in parallel spaced relationship with respect to each other and with alternate opposite lugs in fac to face contact, and welded connections between the lugs which are in face to face contact.

22. An electrical resistance unit comprising a supporting structure, a plurality of pairs of complementary resistance elements, each of said elements having a pair of mounting lugs offset in planes parallel to the plane of the remainder of the element and in non-overlapping relationship with respect to the remainder of the element. means cooperating with said lugs for supporting said complementary elements on said structure alternately and in parallel spaced relationship with respect to each other and with alternate opposite pairs of lugs of adjacent complementary elements in face to face contact, and welded connections between the terminal lugs which are in face to face contact. 1 I

23. An electrical resistance unit comprising a supporting structure, a plurality of pairs of complementary resistance elements, each of said elements having a pair of mounting lugs oflset in non-overlapping relationship with respect to the remainder of the element and in planes respectively which are parallel to the plane of the remainder of the element, means cooperating with said lugs for supporting said complementary elements on said structure alternately and in parallel spaced relationship with respect to each other and with alternate opposite pairs of lugs of adjacent complementary elements in face to face contact, said means comprising elongated bolts extending through perforations in said lugs in such manner that welding between lugs which are in face to face contact is interfered with, projections extending from said lugs which are coplanular with respect to said lugs, and welded connections between said projections which are in face to face contact.

24. An electrical resistance unit comprising a plurality of resistance elements each having a pair of offset lugs interconnected by an intermediate fiat resistive portion, each of said lugs lying in a plane parallel to the plane of the resistance portion of the element and being on the same side of a longitudinatplane through the longitudinal axis of said element and normal to the plane of said resistive portion, whereby said elements are unsymmetrical about said longitudinal axis, supporting means for supporting said elements in parallel spaced relationship with respect to each other on said supporting means with at least some of said lugs extending toward adjacent lugs and in face to face contact therewith, said supporting means being associated with said lugs and reducing the surface area available for welding between contact.

DAVID C. WRIGHT.