US2055766A - Slide-wire unit for potentiometers - Google Patents

Description

Sept. 29', 1936. o. H. HUNT 2,055,766

SLIDE WIRE UNIT FOR POTENTIOMETERS Filed June 29, 1934 INVENTOR 02/?0 H. Hu/vT ATTORNEY Patented Sept. 29, 1936 SLIDE-WIRE UNIT FOR POTENTIOMETERS Ozrc H. Hunt, Waterbury, Conn., assignor to The Bristol Company, Waterbury, Conn., a corporation of Connecticut Application June 29, 1934, Serial No. 733,086

1 Claim. (Cl. 201-62) The invention relates to electrical measuring and/or controlling apparatus of the potentiometer or null type and more particularly to the slide-wire unit customarily utilized therein and comprising a length of resistance wire or slidewire element and a slider or contact-making member adapted for sliding engagement thereover.

The ideal slide-wire for a self-balancing po tentiometer, for example, consists of a bare wire of resistance metal so disposed that the sliding contact may engage any point on its length. Owing, however, to the physical diftlculties encountered in obtaining a suitable resistance value within the available dimensional limits, it is seldom that this ideal can be attained with out recourse to relatively complicated mechanisms, which themselves tend to introduce errors ofisetting much of the precision attendant on the use of the continuous bare wire.

In potentiometers of the self-balancing type it is usual, therefore, for the slide-wire to take the form or" a continuous winding on a mandrel or card, a strip of the wire along one edge being left bare for contact with the slider at successive turns, as hereinbefore set forth. By the use of a suihciently great number of turns of wire, the potential drop between successive convolutions may be made so small that a single step from turn to turn, or a momentary shortcircuit between turns is imperceptible in its effect on the galvanometer of the potentiometer.

It will be appreciated, however, that in a slide-wire of this nature, there are certain limitations, and that a compromise must frequently be made between the most desirable resistance value and the size of wire suitable for the purpose of contacting. In order to have a smooth potential gradient there must be employed a relatively great number of turns, calling for a fine wire and a consequently high resistance value. While the total ohmic value of the slidewire circuit may be reduced by placing a suitable resistance spool in parallel with the slidewire, the resistance of the galvanometer circuit is not thereby reduced in proportion, and sensitivity will consequently be sacrificed. Again, slight irregularities in the winding, or particles of foreign matter upon the surface of the wire, may affect the contact resistance or may introduce actual open circuits into the galvanometer circuit.

It is an object of the present invention to overcome the aforesaid limitations and to provide a slide-wire unit which shall be positive in operation and of improved reliability against errors or failure due to the contact resistance or to open circuits; also, a unit which shall be of rugged and rigid construction.

The invention has for a further object the 5 provision of a resistance element which, while retaining the advantages of a large number of contact points, shall be adapted to a low-resistance potentiometer circuit.

In carrying out the invention, the slide-wire 10 structure is subdivided into a plurality of circuits electrically in parallel, and having their respective sliding contacts at the same potential, a single slider or contact-making member, for example, bridging the parallel-connected 15 slide-wires.

By this expedient, not only is the total resistance reduced to a value lower than practicable with a single slide-wire, but the probability of trouble due to contact resistance, open circuits, or irregularity of winding is reduced in direct proportion to the number of parallel circuits.

The nature of the invention, however, will best be understood when described in connection with the accompanying drawing, in which:

Fig. 1 illustrates, in perspective and more or less diagrammatically, a potentiometer assembly illustrating the construction and arrangement of the novel slide-wire unit therein.

Fig. 2 is a detail view, on an enlarged scale, of a suitable form of the slide-wire unit; and Fig. 3 is an end view thereof.

Referring to the drawing, more particularly Fig. 1 thereof, two calibrated slide-wire elements, as the cards It and II bearing the respective windings l2 and I3, are indicated as disposed adjacently and substantially parallel to each other and to be out of electrical contact except at their respective ends where they are joined together by the wires l4, l5 and l6, [1. These ends are then electrically connected into the usual potentiometer circuit including a battery 26 which supplies current to the regulating rheostat 2! through the two parallel connected slide-wire elements, whereby the potential drop along both of said elements is the same and any two opposite points on the respective cards will be at the same potential.

A source of controlling electromotive force or force to be measured, such as that derived from the thermocouple 25, is arranged to be applied to one end of the combined slide-wires and to one side of a galvanometer 26, the other side of the galvanometer being connected to two slider elements or contact members 21 and 28 adapted 55 for engagement with the corresponding windings l2 and I3 for completing the potentiometer circuit.

' It will be understood that these slider contacts, corresponding in number to the individual slide-wire elements, will be electrically connected together in such a manner that there can exist no potential between the various contacts. The latter are supported upon a movable carriage 30, flexible leads 3! and 32 being included in the connection to the contacts 21 and 28 to admit of reciprocation of the said carriage. This may be effected from a lead screw 33 engaged by a nut of the carriage, the lead screw being under control of a servo-motor fi l geared to the said lead screw and in turn under control of the galvanometer 26. For example, the control may be efiected through the contact system 35 actuated under conditions of unbalance of potential in the galvanometer circuit which cause deflection of the galvanometer in the sense to afiect the motor so that the carriage 30, together with contacts '21 and 28 borne thereby, is moved by the lead screw 33 in a direction as to introduce into the galvanometer circuit from the slide-wires a pote'ntialtendin-g tobalance this circuit and to return the galvanometer to its neutral position all of which is well understood and forms no particular part of the present invention. The carend portion of each card and the said bar 32.

One or these bolts is placed at each end of the card; and there is spaced along the bar 42, more or less uniformly, a number of hollow posts 46 of insulating material and of a diameter substantially equal to the space between the windings. r

7 Supported by these posts are 'U -shaped clamping members 4'! embracing the upper side of the windings, being held in place by screws 48 passing through the hollow posts 46 and tapped into the bar 42. This arrangement aiiords a structure which is rigid against lateral distortion as well as against bending stresses due to the contacts bearing upon the lower edges of the said windmgs.

Directly beneath these windings and carried upon rails as and 50 is mounted to run a carriage 5! which bears two mechanically independent sliding contacts 52 and 53 for engagement with the lower edges of the winding which are left bare for securing electrical contact thereto. Leaf springs 54 and 55, resting upon the body of the carriage are caused to bear against the contacts 52 and 53 to hold the same in proper engagement with the respective windings.

It is understood, of course, that the two windings are exactly similar; and it will be appreciated, furthermore, that the total slide-wire resistance in this novel arrangement is less than that of a single slide-wire so that the resistance of the galvanometer circuit may be reduced to the same degree as that of the slide-wire circuit. Moreover, errors in measurement, due to irregurarity in individual units, are minimized; and a high resistance, or open circuit, .at one contact will not prevent a definite and accurate measurement from being obtained.

I'ola im:

A mounting for slide-wire resistors of the type wherein a continuous wire of resistance, material is wound on a fiat card and adapted to be errgaged on an edge by a sliding 'con'tactor: said mounting comprising an elongated metal bar to the opposite sides of which are affixed two of said cards in parallel relationship, together with clamping means intermediate the extremities of said cards and partially including the same, insulating spacer means located along the one edge of the bar and between the corresponding projecting portions of the respective cards, and bolts passing through the separating means into the bar to secure said clamping means thereto whereby the clamping means will resist edgewise strain on said-cards, and obviate distortion of the same.

OZRO H. HUNT.

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