US2163644A

US2163644A - Variable inductance

Info

Publication number: US2163644A
Application number: US31823A
Authority: US
Inventors: Ware Paul
Original assignee: Individual
Current assignee: Individual
Priority date: 1935-07-17
Filing date: 1935-07-17
Publication date: 1939-06-27
Anticipated expiration: 1956-06-27

Description

June27, 1939. P. WARE 2,163,644

VARIABLE INDUCTANCE Filed July 17, 1935 2 Shets-Sheet l llliffliji H lNVENTOR P904 #4905 m3 S ATTORNEYS June 27, 1939. P. WARE VARIABLE INDUCTANCE Filed July 17, 1935 2 Sheets-Sheet 2 un if 47 INVENTOR 904 /%9/? J M/ZW u mm u ATTORN EYS Patented June 27, 1939 UNITED STATES PATENT OFFICE VARIABLE INDUCTANCE Paul Ware, New York. N. Y.

A plication July 17,

5 Claims.

This invention relates particularly to radio receivers and is specially applicable to so-called all-wave sets, although various features of the invention may have other applications.

The tuning of a resonant radio circuit is usually accomplished by varying the capacity associated with a fixed inductance. With a given variable capacity, different ranges may be tuned through by selecting the correct values of such fixed inductances. The variable capacities, or condensers, must stand long mechanical use and cause imperceptible electrical noise when operated in sensitive receiving sets. The tuning ratio of these variable condenser systems now in common use is about 3 to l. The frequencies of 19,000 to 535 kc. covered by a so-called all-wave set therefore requires four different frequency ranges, and hence four different values of fixed inductances per tuning circuit. The variation in wiring, tube, coil and minimum variable condenser capacities makes necessary the use of a number of small trimming condensers to enable the necessary accurate alignment of the several tuning circuits in the manufacture of the allwave receiving set. Before the present variable condenser tuning vogue, tuning was done by varying the inductance associated with a fixed capacity. The variable condenser method however became more economical and eificient than the older inductance tuning method and lent itself better to the reasonably accurate gauging needed in modem multi-range and multi-stage circuits.

One object of the present invention is to provide an improved inductance method of tuning which will possess all of the advantages of the prior variable condenser tuning types with reduced cost of manufacture. Another object is the reduction or elimination of trimmers. Another object is to increase the tuning ratio, thereby reducing the number of tuning ranges re-' quired for the wide frequency coverage and resulting in operating simplicity. Another object is to broaden or narrow one or more predetermined frequency ranges. Another object is the elimination of one or more ranges with resulting elimination of manual switching in the socalled skip-band sets. Other objects and advantages will be understood from the following description and accompanying drawings which illustrate preferred embodiments of the invention.

Fig. 1 is a side elevation of the tuning control; Fig. 2 is an enlarged bottom view of the slideable tuning carriage;

1935, Serial No. 31,823

Fig. 3 is an end view thereof Fig. 4 is a side view thereof;

Fig. 5 is a. diagram of connections of a receiving set embodying the invention;

' Fig. 6 is a side elevation of a modified form of .tuning control; and

Fig. 7 is an end view thereof.

- Referring to Fig. 1, a two-unit variable inductance embodying certain features of this invention is shown. These units are alike and mechanically coupled together by the Bakelite shaft- I which is supported in the ball-bearing assemblies indicated by the blocks 2, 2 and rotated by the wheel and knob 3. In place of the Bakelite shaft shown, a metal shaft with insulating coupling would suffice.

Each variable inductance unit consists of a bare wire solenoid winding 4 or 4a made rigid by winding in a shallow helical groove cut or formed on the outer surface of a Bakelite cylinder 5. Each unit also possesses a grooved contactor wheel l8 that rides along under compression on the wire as a track. The wheel I8 is in continuous contact with winding 4 or 4a touching it at one contact point. Each contactor is mounted in a carriage 6 which is forced by the wheel and wire engagement to move along its fixed guide 1 whenever the'coils are rotated. Rotation of the coils thus varies their effective lengths and hence their inductances. The low potential or ground lead to each unit is connected to the guides I by lugs 8, 8. Connection to the other sides of the variable inductances is through slip-rings 9 and brushes l0 mounted on brush holders II.

The Bakelite cylinders are hollow, open at one end and closed and supported at the other by Bakelite tight fitting inserted disks l2, I2 turned down to a smaller diameter to form supports for the slip-rings 9, 9 to which the nearest ends of the windings 4 and 4a are connected. The in side diameter of the end disks l2 form a tight fit on shaft I and the cylinders 5 are fastened thereto by pins l3, IS.

The metal angular frame ll supports the assembly, together with an upright l5 of Bakelite. The right hand inductance unit has inserted inside it a non-rotating primary winding IS on another hollow Bakelite cylinder I to attached as shown to the support IS, the shaft l passing through its center. The coil I6 is in inductive relation to the variable inductance coil 4a. Each variable inductance may contain 50 turns, thus requiring 50 rotations of the wheel and knob 3 to go from maximum inductance to zero of the coils 4, 4a., or vice versa. The windings 55 may be one inch in diameter, three inches long, and may have an inductance each of 18.7 microhenrys. The winding pitch is not uniform throughout the length of the coils as indicated in Fig. 1, and there may be roughly eight-tenths of the inductance or 40 turns in two inches, and two-tenths or turns in the remaining one inch of the total winding length. The block I? contains a Geneva stop gear for mechanically preventing the system from turning beyond the end limits of the 50 turn coils.

The wheel and knob unit 3 is constructed as shown to enable slow tuning by applying the hand to the large wheel, or rapid tuning by spinning the shaft at the knob. The wheel is a casting and made heavy near its periphery for the purpose of developing momentum when the coils are spun. About four smart spins will carry the carriage mechanism 6. The guide I has its center line parallel to the axis of its coil 4 or 41:. The carriage is composed of a plate l9 having strips l9a secured to opposite sides of its upper face. These strips are grooved on their inner edges which face similar grooves in the edges of guide i. Balls are located between thesegrooves,

and retained in place by pins 21, permitting-free movement of the carriage along the guide. A pair of studs 23 are secured to the plate l9 and extend downwardly therefrom and slidably carry-"-- This truck on their lower portions a truck 2|. has a crosspiece uniting the ends which slide upon the studs 23 and is yieldably presseddownwardly by springs 24 encircling the studs between I the truck and the plate l9. On the lower central.

face ofthe truck is secured by. screw 26, a bearing bracket-25. This bracket is U-shaped with its sides extending downwardly to journal shaft l8a of the contact wheel l8. By loosening the screw 26, the bracket may be turned to such an angle as to give its best average position to permit the contact wheel Hi to readily travel along the wind- 7 ing 4 or 4a, regardless of the varying pitch of. the winding. The screw 26 may then-be .tight-v The ened to hold the bracket in fixed position. contact wheel I8 is formed with bosses l8b'on its sides which have reduced shoulder end bearings against the inside of the sides of the bracket which minimizes the friction and play of the parts. The shaft l8a has a driving fit with the contact wheel and extends beyond the sides of the bracket 25. The outer ends of the shaft are engaged by

brushes

28, 28 which are fastened to the truck and bent to be under pressure against each end of the steel shaft l8a. This gives a bonded contact between the contact wheel l8 and the truck 2|. Electrical connection between the truck and the carriage is insured, or bonded by the flexible lead 29. Electrical connection or bonding between the carriage and the guide I is insured by a leaf spring 30 attached to the upper face of the plate I9, as shown in Figs. 3 and 4.

This bonding is important when using an assembly of metal parts which are relatively movable for radio tuning purposes, especially if the shorter waves are to be tuned. This is necessary to prevent noise while tuning, such noise being caused by poor or intermittent contact of the parts in or near the sensitive circuit of an amplifier. For securing the best results the contact wheel It! should be made of sterling silver. The bearing bracket 25 may be made of brass. The brushes 28 should be of sterling silver and shaft 18a should plated with rhodium. The leaf spring 36 should be made of sterling silver.

Fig. 5 shows the variable inductances connected to form resonant tuning circuits in a superheterodyne receiver. The bare wire rotatable coils or windings l and do have one end of each connected to its slip-ring 9 on which presses a brush ill which forms the terminal for each unit. as diagrammatically indicated in Fig. 5. The guides 'i are grounded as indicated. The non-rotatable winding I6 is indicated in Fig. 5 and is inductively related to its associated variable inductance and is here used as an oscillator tickler. The upper and lower portions of Fig. 5 respectively represent the radio frequency antenna tuner and the oscillator tuner. The parts are inter-connected by a five-pole double-throw switch and the different portions of this switch are segregated and the two positions of these switch parts are indicated by the roman numerals I and II. Thus for the two tuning ranges of the embodiment shown, the switch parts will be thrown to position I for operation over one range and will be thrown to position II for operation over the other range. In switch position II, the range of this particular example is 535 to 3540 kc.; andin position I. the tuning range is 3480 to 19,000 kc. In serieswiththecoils 4 and do. are the inductances 32' with the upper coil 4 and 33. and 34 with the .lower coil4 as deter mined by the switch position. These inductances Apentagrid tube 35 is indicated in Fig. 5xand may be of the type 6A7 or 2A'7 now incommon use. This tube performs thefunctions of oscillator andmixer in the input of superheterodynes.

This tube is employedin-thegusual mann'erfhere except that'the circuits-have'been adapted to the present inye'ntionr The tube 35 comprises the.

control grid 36. the screen grid 31, the plate. 38;,

the filament 39 heated by a source of voltage such as the battery 3l ,qthe cathode40, the oscillator grid 4| and the oscillator-anodegrid 42. The re-,' sistance 43 furnishes the minimum control grid 43. The, capacity 45 serves-as a blocking conbut permitting the passage of radio frequency current to thecontrol grid 36.

same time conveys through. the usual connection, the desired automatic volume control bias to the control grid 36. The capacity '41 acts to block direct current from the oscillator grid- .41; and

the resistance 48 supplies the desired oscillator bias through the drop caused by the current" To the output plate 38 of thetiibeis: attached the intermediate frequenc'ytuned pri therein.

bias; and the capacity 44 by-passes the resistance denser, preventing the passage of direct current The resistance 46 acts as a radio frequency choke and at the plifier and automatic volume control tube and its associated circuit. The tube 53:: may be of the well-known type 75 and is shown in a commonly used circuit making a detailed description unnecessary. The parts enclosed by the rectangle 53 indicate the source of automatic volume control voltage which by way of wire- 54 is applied through the resistor 46 to the control grid 36 of the tube 35. The capacity 56 conveys audio or signal frequency to the usual additional amplifier and signalling indicating device. It will be understood that instead of using the tube 35, two tubes may be substituted therefor in the usual manner, one tube being the first detector or mixer and the other tube the oscillator.

Referring to the. antenna input circuit in the upper part of Fig. 5, when 'the' double throw switch is in position 11, the rotatable coil 4 which may be of 18.7 microhenrys, is in series with a part of the end inductance 32 and resonates with the fixed capacity 51 which may be of .00462 microfarad to tune the input circuit of the tube through a range of.535 to 3540 kc., a ratio of 6.62. The inductance of the part of the end inductance 32 used on the range of switch position II, plus the effect. of leads, may be .44 microhenry at 3540 kc. The incoming signal frequency from the antenna I58 passes through the capacity 14 and through the resonant trap composed of coil 62 and capacity 6| to the tuning circuit 4 composed of part of inductance 32 and capacity 51. The variation in pre-selector circuit tuning is caused by the rotation of the upper coil 4 which causes the grounded contactor l8 to travel along the outside of the coil 4, as already described. The trap 6|, 62 excludes interference from signals .in the region of the frequency of the intermediate frequency amplifier 52 which for these particular constants is tuned to approximately l The oscillator tuning circuit is tuned in unison with the antenna tuning circuit just described but always at a frequency differing therefrom by an amount equal to the intermediate frequency. In switch position II, the oscillator is always on the higher frequency side of the antenna circuit tuner and hence covers a range of 1025 to 4030 kc. As the two variable inductances are alike, this difference in tuning range is accomplished by using a fixed capacity 58 which may be of .00285 microfarad and by adding the shunt fixed padding inductance 59 as shown. The oscillator tickler or feed-back coil for this range consists of the fixed inductance l6 and the fixed inductance 60. The inductance I5 is in inductive relation with the coil 4a; and the inductance 60 is in inductive relation with both inductances 53 and 34, the direction of the tickler windings being such as to cause oscillation. Thus the tickler circuit is from the anode grid 42, through coil I5 which is inoperative on range II, through coil it, through coil 60 and thence to the positive terminal of a by-passed voltage supply as 15.

when the switch is changed to position I, the oscillator is arranged to operate on the lower fre quency side of the antenna tuning circuit and at the necessary 490 kc. separation therefrom. When the oscillator operates on a frequency below that of the antenna tuning circuit, better signal to image and signal to noise ratios appear to be obtainable on the shorter wave lengths. With the antenna range from 3480 to 19,000 kc., the required oscillator range is from 2990 to 18,510 kc., a ratio of 6.22. This oscillator range is covered by the coil in series with the end inductance 33 and with fixed capacity 63 which may be of .0001 microfarad. 'Ihe tickler circuit for range I comprises coil I3 which is coupled to the end inductance 33 and also includes coil it which is coupled to the variable inductance 4a, a large capacity 61 which may be of .005 microfarad serving to short circuit the eilect of coil 60, although utilizingthis codes a conveyor of direct current voltage from the source 15. The winding 16 is wound in such a direction relative to the coil 33 as to cause oscillation. The end inductances 33 and 34 and their leads are approximately equal but they are not combined as one because on range 11 the coil 34 must couple with the coil 60 to produce oscillations when the variable inductance of the coil 4a is very small and there is then very little coupling between the coil i6 and the coil 4a, whereas on range I the end inductance 33 must couple with the coil 16 to cause oscillations when the inductance of the coil 4a is small and at the same time must be away from windings 60 and" in order to avoid excessive losses on the high frequency range I.

The antenna tuning on range I is accomplished by the coil 4 in series with the coil 32 and the fixed capacity 64, the restricted range for tracking with the oscillator being accomplished by the addition of the shunt inductance 65 and by making the capacity 64 of smaller value than capacity 63. The antenna circuit is inductively coupled to the coil 65 by the coil 66. The capacity 64 is the only aligning adJustment required on the entire input system and with high accuracy of manufacture this might be omitted as its value is well above the remainder of the circuit capacity. In this system especially where small coils are used, slight capacity variations are noticed, if at all, at the low frequency end of the range instead of at the high frequency end as in capacity tuning. The accuracy of the inductances is very important at the high frequency ends of the ranges. The inductances of the leads about equals the end inductances and their sum may be about .43 microhenry in each circuit. The reason for tapping coil 32 on range II is to aid tracking by compensating for the dissimilar inductances of the lead lengths and other dissimilarities. Two variable inductance coils having the same total number of turns but differing in their winding patterns at the high frequency end, would be an additional tracking aid.

Over most of range II the oscillator feed-back coupling is between coils 60 and 59, the coupling between 34 and 60 being needed only at the high frequency end. Coupling between coil l6 and coil 40 is helpful on this range, the more turns on coil it the better. The presence of the five or six turn coil 16 is of no importance on this range. On range I, however, coupling between coils I6 and 33 is required at the high frequency end, the coupling between coils i6 and 4a; furnishing feed-back for the balance of range I. The size of coil i5 is limited, as the natural frequency of the range I tickler circuit must be sufficiently higher than 19,000 kc. to void mistracking in that vicinity. This efiect is minimized by increasing the coupling between coils l6 and 4a while keeping the inductance of coil I6 constant. A method for doing this will be described later. For the one inch diameter solenoid coil 4a, a tickler coil diameter of three-quarters inch and of twelve turns two and one-half inches in length may be used.

Figs. 6 and 7' show a modification 0! one of the higher voltage.

units of Fig. 1 wherein the rotatable coil is shaped like a truncated cone. The winding is put on in the same pattern as previously indicated, that is, it may have 10 turns in the first inch of winding at the right end and 40 turns in the remaining two inches of axial winding length. The form 68 on which the coil Ida. is wound may be of Bakelite. The carriage 6 is the same as previously described and travels along the guide I which is parallel to the side of the cone and in the plane of the cones axis instead of being parallel to the axis of the coil. The structure is otherwise the same as described with reference to Figs. 1 to 4; and a two coil unit would be a repetition of the coil and contactor carriage added to the left. One advantage of the conical shape is that it gives more frequency change to a turn at the low frequency end of travel than the cylindrical form, while the desirable high frequency end spread remains substantially like that obtained with the cylindrical form. Another advantage is that the primary or tickler coil on form 69 may be closer coupled to the variable coil l4a for a given degree of mechanical clearance thus tending to avert oscillator tickler circuit resonance at the high frequency end of the high frequency range disturbing the tracking there.

The inductance, with a cone-having diameters of one inch and two inches at the ends, or an average diameter of 1.5 inches, is about doubled compared with the cylindrical form of Fig. 1, assuming the same winding length and number of turns in each case. Consequently the fixed capacities required for tuning for all-wave coverage over the two ranges, are smaller than required for the cylindrical formof Fig. 1 and hence the resonant circuits develop a desirably However, as it may be desired to reduce the number of turns for ease of tuning without materially changing the high frequency broad tuning characteristic of Fig. l, the conical shape offers a winding design of approximately19 microhenrys when wound with 30 turns over a total axial length of two inches, keeping the mean diameter of 1.5 inches and spacing the first 10 turns at the right over one inch and the balance at the rate of 20 turns per inch. The shorter length of this 30 turn unit is of practical importance. The circuit of Fig. 5 applies whether the variable inductances are conical or cylindrical in shape.

Figs. 6 and 7 show how the left-hand guide Ia may be attached to the upright l5 by means of a machined piece 13 which supports the guide la. and which is in turn secured to the upright. It will be noted that the guide I in this modification extends beyond the support l5. This overlapping of the guides l and la saves about one inch in length of a two unit assembly and about one and one-half inches in a three unit assembly. Overlapping of the guides in the cylindrical coil form of Fig. 1 may be accomplished by staggering the fixed positions of the guides at differing angles around the coil axes, but in such a modification it is important that the coil forms be is a U-shaped frame 18 which straddles the guide I and also the carriage 6 when the latter is moved to the end of the guide. The support 18 supports a block 19 in fixed position which in turn supports a series of switch elements to the rear ends of which the circuit connections may be made. The front portions of the switch elements are flexible and carry contacts adapted to engage each other for certain connections in normal position and to form another relationship of connections when operated by the carriage 6. For this purpose, the lower or one of the lower contact elements may carry a roller 12 extending into the path of a cam H carried upon the top of the carriage 6. The contact elements may be initially set to assume certain positions and some of them may have intervening insulating blocks 8| to prevent adjacent contacts from engaging each other and arranged to give the desired change of connections when passing from one range of control to another. In the particu lar case considered, the cam H will come in contact with and push up the roller 72 of the switch assembly at a point where the variable inductance has dropped to approximately eight microhenrys. This point is approximately 35% of the axial winding length from-the right-hand end of the coil. The coil data may be as follows: Axial length 3.2 inches, left end diameter 2.5 inches, small end diameter 1 inch, and a total of 55 turns made of 9 turns at a pitch of .1 inch starting at the right-hand end and with a balance of 46 turns at a pitch of .05 inch. Two such coil units are required to tune the antenna and the oscillator of a superheterodyne circuit as shown in Fig. 5. The switch 80 comprises four doublepole double-throw portions; two portions being side by side with two tiers such as shown in Fig. 6. The switch used on the accompanying tuning unit comprises two such portions, making six portions in all. Five of these are utilized like the five portions shown in Fig. 5 and the sixth portion may be used to shift the illumination from a long to a short wave calibrated scale.

The circuit arrangement and constants already described for range I of Fig.5 apply also to this skip-band range 6000 to 19,000 kc., excepting the increased size of the variable inductance 4 to 65.2 microhenrys, the termination of tuning at 6000 instead of-at 3480 kc. calling for no modification. For the 535 to 1500 kc. range, the previously described circuit of range II applies together with the constants given, except the increase in size of the variable inductance to 65.2 microhenrys. the decrease in capacity 57 to .00134 microfarad, and alterations in the inductance 59 and capacity 58 in order to track the oscillator tuning over this range. The inductance 34 becomes unnecessary but as it is a part of an assembly including capacity 58 and inductances 59 and 60, it may be left in the circuit.

It is apparent that the switch device 80 and actuator 12 could be made smaller and that two or more such devices could be arranged to be actuated by the movement of the carriage at predetermined points and thus permit two or more ranges to be skipped instead of one as has been above described.

Instead of using a rotating trolley or contactor wheel I8, a sliding contactor or shoe may be substituted. In the structure previously described a sliding contact may be obtained by bending the springs 28 inwardly to cause sufiicient pressure tion of the contactor wheel I8; The rotating contactor wheel has the advantage of offering less friction when spinning the coil as above described by means of the knob 3 of Fig. 1, but the sliding contact has the advantage of being self-cleaning. A rotating contactor wheel of silver alloy used in conjunction with a copper wire coil heavily plated with fine silver may give satisfactory results if the mechanism is kept free of dust by proper enclosure. When using a sliding contact, unlike metals having a natural lubricating eifect are desirable. Such a combination is a shoe of silver or gold, or certain alloys of either one, sliding on a rhodium plated wire. Rhodium is very hard and gives excellent wearing qualities and at the same time it is practically unaffected by the atmosphere. In such a combination the shoe wears out first which makes it easy to service the set by merely replacing the shoe. The combination of two precious metals, the harder one plated on the wire and the softer one forming the shoe, presents a very low sliding friction; and a sliding contact has the advantage of self-cleaning. The plating metal on the wire or coil must have a comparatively low electricalresistance in addi tion to presenting a bright clean surface to the contactor shoe or contactor wheel. The slip-rings 9 and brushes I0 should be such as to add no appreciable electrical resistance to the circuit as they are in the resonant circuits and should also have low friction to rotation. A rhodium plated brass slip-ring and a silver or gold alloy brush lightly lubricated is satisfactory and durable. The use of a contactor wheel as a non-rotating sliding shoe has the advantage of being readily turned to a new position and thus affords a number of new contact surfaces.

As rhodium is too hard to be buffed, it is necessary to thoroughly clean, polish and buff the wire and the slip-ring before plating them with rhodium. In case the wire or the slip-ring be made of copper or brass, the base metal is cleaned, bufled, polished and nickel plated before being plated with rhodium. This process results in a bright, hard, smooth rhodium surface. Lubrication when used should be light and a tight enclosure is desirable to avoid any accumulation of dust.

Where an amplifier vacuum tube is placed in circuit between two of the variable inductance tuning units, electrostatic shielding is necessary to prevent undesired oscillations as in any system. Such shielding will also prevent undesired pickup. When using one of these tuning units in the p1ate circuit of a vacuum tube for radio frequency amplification, the coupling from the plate to the resonant circuit may be accomplished by the non-rotating coil IE, but as there is considerable clearance between the coil i6 and 4 in Fig. 1, higher impedance is obtained when the resonant circuit is connected directly across the tube output.

Under certain circumstances, as where larger variable inductances are used especially if shielded, it is necessary to ground the unused part of the variable coils, in which modification it is desirable to insert a small fixed inductance 82, as shown in Fig. 5 in series with the added ground connection to prevent end turn short-circuiting when the contactor approaches the low frequency end of travel. Such added ground connection is made through an additional slip-ring 83 and brush 84.

It isto be understood that the scope of my invention is not to be limited to the particular disclosures made. For instance, the coil instead of rotating against a guided contactor could be made stationary and the contactor mounted on a carriage revolving around the inside or outside of the coil. The side of the coiL'instead of being a straight line, might be in the form of an arc of a circle with the contactor carriage mounted at'the end of a radial arm the other end of which would be able to turn in a pivot located at the center of the circle. It is also obvious that a variable condenser might be mechanically attached to one of the variable inductance units to vary the capacity of a resonant circuitat the same time the inductance is varied. High frequency iron such as that known commercially as ferrocart may be inserted in the coils to increase their inductance, permitting the use of a smaller fixed capacity on the low frequency range and thus desirably increase the resonant circuit gain. Also this invention applies equally well to transmitters as to receivers if the necessary insulating and current carrying requirements are met. It is also obvious that any type of frequency indicating scale may be attached or geared to the carriage or rotating system.

Various other modifications and applications of the invention may be made without departing from the scope thereof.

I claim:

1. A variable inductance device comprising a pair of spaced upright supports, a rotatable shaft extending therebetween and pivoted therein, a hollow coil form surrounding said shaft with its axis concentric with said shaft, said form being rigidly secured to said shaft at one of the form ends, the other end of said form being open and free of support, a bare conductor helically wound on said form and a contactor member guided along the side of said form parallel to said side and arranged to make continuous sliding contact with said helical conductor when the shaft, form and helix are rotated, a second coil form surrounding said shaft and rigidly supported in cantilever fashion on the one of said upright supports at the unsupported open end of said first form and projecting inside said first form, and a coil winding on said second form.

2. A variable inductance device comprising a pair of spaced upright supports, a rotatable shaft extending therebetween and pivoted therein, a

hollow cylindrical coil form surrounding said shaft with its axis concentric with said shaft, said form being rigidly secured to said shaft at one of the form ends, the other end of said form being open and free of support, a bare conductor helically wound on said form and a contactor member guided along the side of said form parallel to said side and arranged to make continuous sliding contact with said helical conductor when the shaft, form and helix are rotated, a second coil form surrounding said shaft and rigidly supported in cantilever fashion on the one of said upright supports at the unsupported open end of said first form and projecting inside said first form, and a coil winding on said second form.

3. A variable inductance device comprising a pair of spaced upright supports, a rotatable shaft extending therebetween and pivoted therein, a hollow.conical coil form surrounding said shaft with its axis concentric with said shaft, said form being rigidly secured to said shaft at one of the form ends, the other end of said form being open and free of support, a bare conductor helically wound on said form and a contactor member guided along the side of said form parallel to said side and arranged to make continuous sliding 7 contact with said helical conductor when the shaft,

form and helix are rotated, a second coil form surrounding said shaft and rigidly supported in cantilever fashion on the one of said upright supports at the unsupported open end of said first form and projecting inside said first form, and a coil winding on said second form.

4. A variable inductance device comprising a bare conductor formed into a helix and means supporting said helix for rotation about its axis a guide bar supported along the side of said helix. a contact member guided by the conductor of said helix and also guided by said guide bar whereby said contact member will travel along the conductor of said helix when said helix is rotated and will be driven by said helix along said guide bar, and an electric switch having actuating means positioned in the path of travel of said contact member along said guide bar so as to be actuated by said contact member at a predetermined position in its travel along said bar.

5. A variable inductance device wherein a coil -formed 01' a bare conductor is supported ior rotation about the coil axis, a guide bar individual to said coil is supported in spaced parallel relation to the side of the coil and a contactor engages both the guide bar and the coil and travels along the guide bar and the conductor of the coil whenever the coil is rotated to vary the inductance of the device, characterized by the fact that said contactor is positioned between said guide bar and said coil and comprises a carriage movable along said bar in contact therewith and a truck having a trolley wheel pinioned therein, said wheel being grooved to the conductor of said coil so as to travel along said conductor as a track when said coil is rotated, and spring means in compression between said truck and said carriage urging said truck away from said bar and toward said coil so as to hold said trolley wheel on said wire and electric conductive means providing an electric conductive circuit between said guide bar and coil conductor.

PAUL WARE.