US2487948A - Proportional cycling switch - Google Patents

Description

NOV. 15, 1949 A. SENN PROPORTIONAL CYCLING SWITCH Filed Aug. l, 1945 Ish Je A. SEN/v INVENTOR.

Patented Nov. 1s, 1949 UNITED STATES PATENT vOFFICE PROPORTIONAL CYCLING SWITCH Jurg A. Senn, Washington, D. C.

Application August l, 1945, Serial No. 608,167

12 Claims. (Cl. 200-136.3)

This invention relates to cycling switches adapted to automatically supply electric power to using devices at varying time intervals and more specifically to those in which the length of the On and Oi cycles depends on both the actual temperature and the desired temperature.

Gouy in the French publication: Journal de Physique 6, 479 (1897) describes a thermoregulator in which a platinum wire is extended downwardly into contact with the mercury meniscus ofV a U-tube which is closed at one end and thus serves as a temperature indicator. The platinum wire is oscillated vertically by an oil-center cam into and out of the mercury to control the energization of the electric heating circuit for the temperature indicator. The connection to his heater is such that energy is periodically supplied to the bath during that part of the cycle of motion of the platinum wire duringwhich the latter is not in contact with the mercury. lAn increase in the temperature of the mercury bulb results in the meniscus rising which causes a decrease in the length of time during which energy is periodically supplied to the bath, thus tending to stabilize the bath temperature.

One of the most serious, objections to Gouys oi-center cam type of switch contact actuator is the variable spacing of the contacts in their open position. This is most objectionable when the platinum wire is withdrawn from the mercury meniscus for such a slight distance that the circuit across the contacts is not opened but continues as an arc, burning and pitting the oo ntacts. In Gouy this variation in the spacing of the contact surfaces due to burning does not change the adjustment since one of the contacts (mercury) is a part of the temperature-responsive element. In d'isc or button-contact switches using 'the Gouy arrangement, however, contact spacing is one of the factors afiectingtemperature calibration and any reduction of this space necessitates recalibration of the device.

Since the eccentricity of the cycling cam should wherever possible due to the risk of shorting through broken insulation, breakage due to excessive twisting, etc. The present invention eliminates each of the foregoing disadvantages and in addition provides further advantages which will be described.

Therefore, it is an object toprovde a cyclic switch operating on the Gouy principle but modied to provide contacts in which the make and break travel is uniform over the entire range of operation of the switch.

Another object is .to provide a switch of the character specified in which variations in the thickness of the contacts due to wear, pitting, burning or other cause does not affect the calibration of the switch.

A further object is to provide a cyclic switch of the character specied in which the range of stabilizing regulation is not dependent on the eccentricity of an off-center cam, but instead is limited only by the dimensions of a cylindrical cam, thereby greatly increasing the On-Oil, or stabilizing range and making the switch more compact.

Another object is to provide a proportional,

cycling switch utilizing the Gouy principle in a superposed, spring-biased lever system which is virtually free of lost motion or deformation of the parts over a long, switch-operating life.

A further object is to provide a proportional, cycling switch utilizing the Gouy principle in a multiple lever system that is protected from excessive biasing forces by a tensioned, follow-up spring.

A further object is to provide a switch of the character specified having a multiple lever system in which the biasing action of the temperature indicating means as well as that of the temperature setting cam is applied close to the fulcrum of the main lever, thereby lessening the risk of deformation of the levers and consequent be large as compared with the movement necesinaccurate temperature responsiveness of the switch.

Another object is to provide a proportional, cycling switch utilizing a peripheral, circular, disc-cam as the temperature adjustment means thereby obtaining virtually 360 degrees of adjustment in stepless, straightline or other pattern of control as desired.

Another object is to provide a switch of the character described that is inexpensive and is easily manufactured, assembled and adjusted.

Further objects and advantages will be apparent as the description proceeds.

Reference is now made to the drawing wherein Fig. 1 is a plan view of a preferred form of cyclic switch embodyinar the principles of my invention, partly broken away along the line I-I of Fig. 2, the switch contacts being omitted for clarity and the cycling cam lever shown partly broken away;

Fig. 2 is a sectional view of my switch taken on the line 2-2 of Fig. 1, the switch contacts and their carrier being shown in elevation and the cycling cam lever of Fig. l being shown in its operative relation to the cycling cam; and

Fig. 3 is an end, sectional View of my improved switch taken on the line 3-3 of Fig. 1, the switch contacts and the carrier thereof being shown in elevation.

In the drawing there is shown at I a box-like,

outer housing on one wall of which is mounted by means of screw 2 and nut 3, a circular cam-disc 4 having an inclined periphery providing a cam surface 4a. Cam-disc 4 may be rotated to any desired setting by means of the combined knob and dial 5. On the opposite wall of the outer -housing I and to one side is mounted the nipple block Ii of the flexible diaphragm assembly and held in place by nut 1 threaded thereon. Block 5 carries the lower wall 8 of the diaphragm as,- sembly while the upper diaphragm wall 9 carries a stud III adapted to transmit pressure from the expansion of the diaphragm walls to a ball II as will later be described. The diaphragm walls are peripherally sealed and are adapted to separate and move stud I Il upwardly upon the ingress of uid through the bore in block 6 as will be understood.

An inner housing I2 adapted to nest within the open end portion of outer housing I is formed with side walls I3 and I4 the latter of which has an inwardly onset, central portion I4a, a top wall I5 and a bottom wall I6 having an upwardly-turned portion I6a. Walls I5 and I6 are formed with oppositely disposed depressions I5b and I6b in which is journaled a hinge pin I1. The latter serves as a pivotal mounting for a bell-crank lever I8 having a centrally disposed, lower ear I9 and an opposite, upper ange 20 which extends across almost the full width of the inner housingl2. One end of a strip 2| of spring steel is united to flange 20 by means of rivets 22 and backingplate 23 and its other end is united to the end of cycling cam lever 24 by rivets 22 and backing plate 26. It will be understood that rivets 22 are repeated at intervals in the superposed portions of the united members (Fig. 1) to give permanent and secure connection of these parts.

Cycling cam lever 24 is generally triangular in shape and is stiiened by the formation of anges 24a in its marginal edge portions. The left end of this lever narrows into a rectangular portion 24h and terminates in a cam-engaging portion 25, which like the flanges 24a is bent downwardly out of the general plane of the cam lever. A cylindrical cam 21 formed by moulding en bloc or by cutting an outer tube 21a to the outline shown in Figs. l and 2 and driving it tightly over a solid cylinder 28 which is mounted on a synchronously drive shaft 29, engages portion 25 of the cycling cam lever 24 during a half revolution of the cam and raises the left end of lever 24 through a distance equal to the height of the cam when the parts are arranged as shown in Fig. 2. A riser block 30 made, preferably of a rigid, insulating material and having a wall 3l of arcuate shape corresponding to its distance from hinge pin l1 is mounted on the cycling cam lever 24 by means such as rivets 32.

Normally closed switch contacts 33, 34 are mounted on a spring arm 35 and a conductor strip 35, respectively, and the assembly suitably insulated from and fixed to the wall I5 of the inner housing as by insulating spacers 31 and any desired means such as screws 38 and others (not shown) which may extend upwardly through insulating spacer blocks 39 from wall I5. I prefer to employ a pair of spaced, conductor strips 3B provided with terminal connections 40 and contacts 34 and bridge these contacts by means of a pair of electrically interconnected, similar contacts 33 fixed to the spring arm 35, thus providing two paths for the electric current 'oetween terminals 43.

Particular attention is directed to the shape of cam 21. While the inner margin of the tube portion 21a may have any desired shape depending on whether a straight line or other relationship is desired between the On and Ol periods, in the preferred form, the tube 21a extends halfway around the cylinder 28 at the longitudinal midpoint of the latter, thus giving equal On and Off intervals at this point. The leading edge 21h of the tube 21 is so inclined as to open the contacts 33, 34 during the optimum time interval, i. e. over at least one-half cycle of the A. C. power supply so that the circuit across the contacts will be broken as the current passes through zero. The inclination of cam edge 21b also corresponds to the slope of portion 25 of the cycling cam lever 24 and is preferably curved about the hinge pin I 1 as a center so that the lifting force of the cam edge 21a is exerted in line with cam lever 24. The trailing edge 21e of the tube 21 is radial to the cam axis to produce rapid closure of the contacts. It is obvious that these functions of tube 21a and cylinder 28 with respect to the operation of the contacts could be interchanged if desired. The position of the lever portion 25 along the longitudinal axis of the synchronously rotated cam will, of course, determine for what part of a cam revolution the contacts 33, 34 are openedand closed. The factors and mechanism which determine the position of this levei` in its scanning movement across the cam will next be described.

The upwardly turned portion ISa of bottom wall I6 is formed with a depression Ib which serves as a retainer for a coil spring 4I the other end of which seats in a depression I8a in the bell crank lever I8 to urge the latter counterclockwise (Fig, l) to the position indicated by the dotted lines |80, and rotate the cycling cam lever 24 to its full On position at the lower end of cylinder 28. This normally 0n position of lever 24 is subject to the inuence of two other factors, viz: (1) the setting of the range dial 5 and (2) the pressure exerted by the diaphragm stud I as will now be described.

A diaphragm lever 42 located centrally of the inner housing I2 and outer housing I has a downwardly-inclined retaining aperture 43 for ball II which is held in its retained position by a at, follow-up spring 44, the remote end of which is iixed to an intermediate portion of lever 42 by means of spacer 45 and rivet 46. A coil spring 41 somewhat weaker than spring 4I is centered beneath the floating lever 42 by the head of rivet 46 and its lower end is retained by a raised portion |3a of side wall I3 to bias lever 42 upwardly. The right end of floating lever 42 is offset downwardly from the portion just described and has fixed therein a hardened, pointed stud 48 in engagement with a hardened, disc-plate 46 fixed in the central portion of a wedge-shaped arm 18d which is normal to the major arm ofbell crank I6 and integral therewith as shown in Fl .1.

lie left end of floating lever 42 is bent downwardly and then sharply up at 50 to provide an angular seat for a pair of knife edges 5I, 52 formed on a pair of downwardly bent, spaced portions oi' a lever extension 53. The latter is xed to a stiff spring strip 54 by means of rivets 55 which secure also a superposed, adjustable lever 56 having a slightly offset, left end 51 contacted by an adjustment screw 58 threaded through a boss 59 formed in the central portion 66 of lever extension 53 and accessible through an aperture (not shown) provided in the closed end wall of housing 1. The bent, spaced portions of lever extension 53 have marginal stops 6l which serve as guides for the floating lever 42. Rivets 62 and backing plate 63 serve to unite the strip 54 to the offset portion |4a.

It will be seen that lever 56 and its extension 53 provide a fulcrum constituted by the knife edges 5I, 52 and adjustable vertically (Fig. 1) upon rotation of cam disc 4 and also easily calibrated to its zero setting by means of adjustment screw 5 8. Floating lever 42 is thus biased upwardly by spring 41 so that stud 48 always contacts disc plate 49, but since spring 41 is weaker than spring 4I, the latter will rotate cycling cam lever 24 clockwise to its full On position with respect to cycling cam 21 until the remotely located, thermal feeler or pressure-responsive member (not shown) causes the diaphragm 8 to expand, moving the stud ID and lever 42 upwardly, thereby slightly rotating cycling cam lever 24 in a clockwise direction toward the Off position.

The operation of my switch for electric range use, for example, is as follows: Assume that terminals 40 are connected in series with the `power supply to energize an annular range heating unit and that nipple block 6 of the flexible diaphragm 8 is connected by a suitable conduit with a thermal bulb or feeler member located centrally of the heating unit as is shown, for example, by the Weber et al. Patent No. 2,303,012. The knob 5 is turned to the desired temperature setting for the particular cooking operation from its Oil? position wherein the highest and preferably stepped portion of cam 4a presses downwardly on lever 51 to maintain cycling Acam lever 24 in its Off position at the upper end of cam 21. Rotation of cam 4a raises fulcrum knife edges 5l, 52 so that spring 4l acting through bell crank I8 moves the cycling cam lever to full On position and moves the right end of lever 42 downwardly about the edges 5|, 52 as a fulcrum since nipple block 6 in the cold position of diaphragm 6, 9 is ineffective as a fulcrum. As the heating unit raises the temperature of the cooking vessel to the setting of dial 5, the upward movement of diaphram 6, 9 and of stud l0 raises the ball l l and through it, the follow-up spring 44 and the right end of lever 42 to rotate the cycling cam lever 24 clockwise to an intermediate position on the cam 26 in which the heat supplied just equals the heat losses of the heated system.

In addition to its function of transmitting movement to the lever 42, follow-up spring 44 has the right end thereof upwardly (Fig. 1) and its follower portion 25 against the straight, radial wall 21e of cam 21. The top of ball Il will flex the follow-up spring 44 upwardly suiiiclently to absorb the difference in height of cam 4a that was not utilized in moving follower portion 25 to the radial wall. This condition will continue until the rotation of cam 26 has brought the inclined wall 21a against the follower portion 25 raising the same so that it may snap to the fully Oil!v portion of cam 21. Overshooting is prevented by engagement of bell crank I8 with the casing I6 as shown by the dotted line 64; similar engagement is indicated by dotted line I8c prevents overshooting of the cyclical cam lever 24 in the fully On position.

It will be apparent, of course, that portion 25 could be provided with a laterally'upturned lip so that it would immediately surmount the radial wall 21o of cam 21. 'I'hls arrangement would be inferior to that shown, however, since it would not provide the desired quick closure of contacts 33, 34 as portion 25 drops down over the radial cam wall at 21e (Fig. 1).

In addition to the greatly amplified movement of diaphragm 8, 9 and cam 4a provided by my lever arrangement and resulting in the relatively large, stabilizing excursion of follower 25 axially of cyclic cam 28 with ,consequent temperature control stability, other important features of my novel switch will be apparent. Cam 4 and adjustment screw 58 are in vertical alignment and only slightly offset from the knife edges 5|, 52 so that bending stresses in levers 53 and 56 are minimized. Except for the stepped Off portion of cam 4a the entire 360 degree sweep of this surface may be utilized to spread the temperature range over a large, angular movement. Also the cam inclination need not be constant but may be steep through a narrow angle from the Oif position to a low temperature, say 200 degrees F. permitting more than A1 of the cam length to be utilized for the temperature range most used, from 200 to 500 degrees F. for example. The use of knife-edges and spring-biasing in the lever system eliminates lost motion and assures continued accuracy of calibration. While cam 26 is rotated at a-uniform and preferably, slow speed, say one revolution per minute, as long as any of the four switches grouped thereabout are in On position (indicated by the radial arrows in Fig. 2) the form of switching means for the driving motor of this cam is not a part of this invention since various arrangements for this purpose will be obvious to those skilled in the art.

It will be apparent that the preferred form of my invention comprises'a laterally movable cam lever having a follower portion which sweeps axially across a generally cylindrica1, rotary switch actuating member, the lateral position of the former being dependent` upon the indicated and desired temperatures of the heated body, and the surface of the member being divided into axially variable portions, certain of which together with the follower portion effect closure of the switch contacts and other portions of which similarly effect opening of said contacts. However,

agencias other forms of my invention will be readily apparent.

Hence, instead of utilizing the cylindrical, stepped cam as the actuating means for tie cycling cam lever 24, a rotary member corresponding to the hollow cam 2l' could be used to raise the lever 24 and a transverse stop mounted on the left wall of the housing I (Fig. 2) could serve to limit the lower movement of said lever. Since the foregoing as well as other variations of my preferred form will be obvious to those skilled in the art, they have not been illustrated nor described in detail. However, I do not wish to be limited to the precise construction specifically disclosed as the preferred embodiment of my invention but only as may be fairly within the scope of the following claims:

1. A switch comprising normally closed electric contacts, a rotary cam having an outer cylindrical surface terminating in an axially inclined marginal wall substantially coextensive circumferentially with said surface and also normal thereto and the extremities of which are joined by a wall which is inclined to said surface and generally parallel to the axis thereof, a follower yieldably mounted to alternately engage said walls during the rotation of said cam and to be moved thereby into opening and closing relationship, respectively with said contacts and also pivotally mounted on an axis which is normal to the axis of said cam, pressure responsive means disposed beneath said follower and between said cam and pivotal mounting for moving said follower arcuately across said cam in a direction generally parallel to the axis of the latter whereby the duration of the open and closed conditions of said contacts will be a function of the pressure exerted by said means, and similarly disposed stop means spaced from said pressure responsive means for limiting the effectiveness thereof.

2. A switch comprising electric contacts, a rotary cam having an outer cylindrical surface terminating in an axially inclined marginal wall substantially coextensive circumferentially with said surface and the extremities of which are joined by an arcuate wall which is generally parallel to theaxis of said surface, a follower yieldably mounted at its remote end to urge its opposite end into engagement with said walls alternately during the rotation of said cam and to be moved thereby into opening and closing relationship, respectively with said contacts, said follower being also pivotally mounted at the center of curvature of said arcuate wall, pressure responsive means disposed between said pivotal mounting and said cam for moving the opposite end of said follower about its pivotal mounting and across said cam in a direction generally parallel to the axis of the latter whereby the duration of the open and closed conditions of said contacts will be a function of the pressure exerted by said means.

3. The combination set forth in claim 2 wherein said marginal wall is normal to said cylindrical surface, said arcuate wall is inclined thereto, and the cam engaging end of said follower is provided wth a pair of surfaces having the same inclination as said walls insuring quick closing and opening, respectively of said contacts.

4. A switch comprising electrical contacts, a rotary cam having a circumferentially continuous, outer, cylindrical surface terminating in an axially inclined marginal Wall substantially coextensive circumferentially with said surface and the extremities of which are joined by an arcuate wall which is generally parallel to the axis of said surface, a casing disposed to one side of said cam and including parallel, top and bottom walls and a connecting end wall, contact carrying means carried by said top wall, a vertically disposed hinge extending through said parallel walls at the center of curvature of said arcuate wall, a follower yieldably mounted at its remote end to urge its opposite end into engagement with the walls of said cam alternately during the rotation of the latter and to be moved thereby into opening and closing relationship, respectively with said contacts, said follower being also pivotally mounted on said hinge, pressure responsive means disposed between said parallel walls for moving the opposite end of said follower aboutits pivotal mounting and across said cam in a direction generally parallel to the axis of the latter whereby the duration of the open and closed conditions of said contacts will be a function of the pressure exerted by said means.

5. A switch for electric heaters comprising electric contacts having open and closed positions, an elongated, pivotally mounted cycling lever including a cam follower portion and a transversely disposed, contact actuating portion, a cylindrical cam rotatable about an axis normal to the pivotal axis of said lever and having an axially variable, elevated portion corresponding to one of the positions of said contacts, and an inclined, uniformly stepped portion generally curved about the axis of said lever and adjacent said elevated portion for lifting said follower to the latter portion of said cam to produce uniform relative movement of said contacts, means including a relatively strong compression spring biasing said lever about its pivotal mounting in one direction, means including a relatively weak compression spring biasing said lever inthe opposite direction, means responsive to the temperature of said heater for reinforcing said latter spring to move said lever in said latter direction, and a manually adjustable stop actingr in opposition to said last-named means for limiting the movement of said lever in the latter direction whereby the open and closed intervals, respectively of said contacts will stabilize the energy supplied to said heater at the temperature corresponding to the adjusted position of said stop.

6. A switch for electric heaters comprising electric contacts having open and closed positions, a pivotally mounted cycling lever including a cam follower portion and a transversely disposed, contact actuating portion, a cylindrical cam rotatable about an axis normal to the pivotal axis of said lever and having an axially variable, elevated portion corresponding to one of the positions of said contacts and an inclined, uniformly stepped portion generally curved about the pivotal mounting of said lever and adjacent said elevated portion for lifting said follower to the latter portion of said cam to produce uniform relative movement of said contacts, means including a relatively strong compression spring biasing said lever about its axis in one direction, an adjustable lever providing a .shiftable fulcrum adjacent one end thereof, a floating lever having one end fulcrumed thereon and the opposite end in force transmitting relation to said cycling lever, means including a relatively weak compression spring biasing said oating and cycling levers in opposition to said rst-mentioned compression spring, means responsive to the temperature of said heater for reinforcing said relatively weak spring to move said last mentioned levers against the force of said relatively strong spring, and a manually adjustable stop adaptedto engage said adjustable lever to determine the position of such fulcrum.

'1. A switch for electric heaters comprising electric contacts having open and closed positions, a pivotally mounted cycling lever including a cam follower portion and a transversely disposed, contact-actuating portion, a cylindrical cam rotatable about an axis normal to the pivotal axis of said lever and having an axially variable, elevated portion corresponding to one of the positions of said contacts and an inclined, uniformly stepped portion curved about the pivotal mounting of said lever and adjacent said elevated portion for lifting said follower to the latter portion of said cam to produce uniform relative movement of said contacts, means including a relatively strong compression spring biasing said lever about its axis in one direction, an adjustable lever providing a shiftable fulcrum adjacent one end thereof, an apertured floating lever having one end fulcrumed thereon and the opposite end in force transmitting relation to said cycling lever, means including a relatively weak compression spring biasing said floating and cycling levers in opposition to said first-mentioned compression spring, a spring-pressed ball seated in said aperture, means responsive to the temperature of said heater and adapted to engage said ball for reinforcing said relatively weak spring to move said last mentioned levers against the force of said relatively strong spring, and a manually adjustable stop adapted to engage said adjustable lever to determine the position of said fulcrum.

8. A switch comprising normally closed electrical contacts, a rotary cam having an outer cylindrical surface terminating in an axially inclined marginal wall substantially co-extensive circumferentially with said surface and the extremities of which are joined by a wall which is generally parallel to the axis of said surface, a bell crank lever pivotally mounted about an axis normal to the axis of said cam and having an elongated flange extending parallel to the axis of said cam and an arm below said flange and extending toward said cam, an elongated, resilient strip carried by said flange, a rigid, generally triangular cam follower having a base co-extensive with said strip and united thereto and a narrow apical portion adapted to alernately engage said walls during the rotation of said cam and to be moved thereby into opening and closing relationship, respectively with said contacts, pressure responsive means disposed beneath said cam follower for moving the latter arcuately across said cam in a direction generally parallel to the axis of the latter whereby the duration of the open and closed conditions of said contacts will be a function of the pressure exerted by said means.

9. A switch comprising electric contacts having open and closed positions, a-rotary cam having an inner cylindrical surface, a radially spaced outer cylindrical surface, an axially inclined marginal wall substantially co-extensive circumferentially with said latter surface and having terminal portions spaced longitudinally thereof, and a curved marginal wall uniting said portions, said Walls being disposed between said surfaces and serving to define said latter surface, a follower for changing the relative positions of said contacts, said follower being pivotally mounted at the center of curvature of said latter marginal wall about an axis which is normal to the axis of said surfaces and also yieldable radially of said surfaces to alternately engage the latter and to be moved from said inner surface to said outer surface by engagement with said curved marginal wall, and pressure responsive means for moving said follower arcuately across said cam about said pivotal mounting whereby the duration of the open and closed conditions of said contacts will be a function of the pressure exerted by said means.

10. A switch comprising electric contacts having open and closed positions, a rotary cam having an inner, cylindrical surface, a radially spaced, outer cylindrical surface, a radially disposed, axially inclined, marginal wall substantially co-extensive circumferentially with said latter surface and having terminal portions spaced longitudinally thereof and a curved, nonradial marginal wall uniting said portions, said walls being disposed between said surfaces and serving tov deilne said latter surface, a follower for changing the relative positions of said contacts, said follower being pivotally mounted at the center of curvature of said latter marginal wall about an axis which is normal to the axis of said surfaces and also yieldable radially of said surfaces to alternately engage the latter and to be moved from said inner cylindrical surface to said outer cylindrical surface by engagement with said curved, marginal wall, /and pressure responsive means for moving said follower arcuately across said cam about said pivotal mounting whereby the duration of the open and closed conditions of said contacts will be a function of the pressure exerted by said means.

11. A switch comprising normally closed electric contacts, an elongated arm carrying one of said contacts and having an overhanging portion, a rotary cam having an inner cylindrical surface, a radially spaced outer cylindrical surface, an axially inclined marginal wall substantially co-extensive circumferentially with said latter surface and having terminal portions spaced longitudinally thereof, and a curved marginal wall uniting said portions, said walls being disposed between said surfaces and serving to define said latter surface, a follower pivotally mounted at the center of curvature of said latter marginal wall about an axis which is normal to the axis of said surfaces and also yieldable radially of said surfaces to alternately engage the latter and to be moved from said inner surface to said outer surface by engagement -with said curved marginal wall, means integral with said follower for moving said elongated arm at any pivotal position of said follower and pressure responsive means for moving said follower arcuately across said cam about said pivotal mounting whereby the duration of the open and closed conditions of said contacts will be a. function of the pressure exerted by said means'.

12. A switch comprising electric contacts having open and closed positions, a rotary-cam having an inner, cylindrical surface, a radially spaced, outer cylindrical surface, a radially disposed, axially inclined, marginal wall substantially co-extensive circumferentially lwith said latter surface and having terminal portions spaced longitudinally thereof and a curved, nonradial marginal wall uniting said portions, said walls being disposed between said surfaces and serving to define saidlatter surface, a follower for changing the relative positions of said contacts and having a cam engaging portion of the Same non-radial inclination as said curved, marl ginal wall, said follower being pivotally mounted at the center of curvature of said latter marginal wall about an axis which is normal to the axis of said surfaces and also yieldable radially of said surfaces to alternately engage the latter and'to be moved from said inner 'cylindrical surface to said outer cylindrical surface by engagement with said curved, marginal wall, and pressure responsive means for moving said follower arcuately across said cam about said pivotal mounting whereby the duration of the open and closed conditions of said contacts will be a function oi the pressure exerted by said means.

JURG A. SENN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PAWS Number Name Date 1,812,897- Owens July 7, 1931 1,962,999 Owens June 12, 1934 2,091,734 Himes Aug. 31, 1937 FOREIGN PA'I'ENTS Number Country Date 353,550 Great I rtain July 30, 1931

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