US1880239A - Resistor - Google Patents

Description

Oct. 4, 1932. G.' B. cRousE 1,880,239

'RESISTOR Filed July 24, 1930 2 Sheets-sheet 1 BYW Patented Oct. 4, 1932` UNITED STATES ,Pari-:Nrl orf-'ICE GEORGE B. CROUSE, F NEWARK, NEW JERSEY, ASSIGNOR T0 HARDWIOK, HINDLE, INC.,

OF NEWARK, NEW JERSEY, A CORPORATION OF NEW JERSEY RESISTOR Application led July 24, 1930. Serial No. 470,380.

This invention relates to resistors and more particularly to those of the tubular, vitreous enamel coatedl type.

' In general a vitreous enamel resistor comprises a tube of porcelain or other refractory material, around the ends of which are clamped suitable contact bands of copper or other vconducting material. The resistance wire, which is usually an alloy having high electrical resistance, is then wound on the tube between the contact bands and riveted or otherwise attached' thereto at its ends.

The wound tube is then dipped or sprayed with the vitreous enamel slip and fired at such a temperature that the enamel becomes molten and flows over the entire surface of wire and tube. One or more coats of enamel, as required, are applied and fired over the original coat.

It is obvious that in a resistor of this type, since the entire wire is coated with enamel, it is impossible to make any ad'ustments of the resistance value or to take o a contact at any point after the resistor is completed. This feature is disadvantageous from several angles. In the first place, even through the original resistor is wound accurately to a specified resistance value, oxidation of the wire will take place in the ring process, as a result of which the wire is reduced in crosssection and its resistance increased by an amount which is not accurately predictable; secondly, in many applications of a resistor it is desirable to be able to change the resistance in service or to use the device as an adjustable potentiometer.

l Many attempts have been made in the past to produce a resistor of the t pe to which this invention is directed, none o which have been conspicuously successful. f Since the finished enamel coating is very hard and brittle, it is impossible to satisfactorily grind it off along the section in which it is desired to expose the wire, and, therefore, most of the previous attacks on the problem were directed to exposing a track before firing. The common method was as follows: Before the wound resistor was dipped or sprayed in theenamel slip, a shield, generally of adhesive tape, was laid l along the resistor, the enamel thus being'prevented from reaching the area which it was proposed to expose. The disadvantages of this method are twofold In the first place, the iiuid enamel in the furnace, during the ring process, has a very high surface tension and it therefore pulls back away from that area which was shielded and leaves a` larger and irregularly edged area exposed. -This produces a unit of very poor appearance and has other disadvantages, as will be seen immediately. Secondly, the wires are entirely exposed and unsupported in the slot. As a result of this, any form of sliding contact that may be used with the wires tends to displace them, bend them, and ultimately to break them.

One object of the invention is a method of producing a vitreous enamel resistor, as well as the resistor itself as an article of manufa cture, having an area in which the wires are exposed, so that contact may be made with them at any desired point in this area. Except where the wires are exposed, they are completely embedded in enamel.

Another object of the invention is the provision with a resistor of the type described above of a slidable contact device whereby the resistor may be used as a rheostat or potentiometer.

Briefly described, the method consists in completely coating with the enamel slip a form on which the resistance wire has previously been wound. For most convenient practice of the method the enamel is allowed to partially dry and the unit is then placed in a 35 device which has a shield which covers all portions of the resistor except the area which it is desired to expose. The slip is then brushed oii of that area, the unit is removed from the device and fired in the usual way. Some pulling of the enamel may occur on the first coat, but on the second coat a clean edged exposed area results, in which only the upper surface of the wire projects from the enamel, the brushing having allowed a portion of the slip to remainbetween the turns.

The particular nature of the invention, as well as other objects and advantages thereof, will appear most clearly from a descriptionof certain preferred embodiments there- 1M device,

Fig. 3 is a transverse sectional view on an enlarged scale showing the exposed portion of the resistor, this view being taken on the line 3 3 of Fig. 4,

Fig. 4 is a corresponding view, but in longitudinal section along the line 4 4 of Fig. 3,

Fig. 5 shows a modified form of tube for the resistor whereby sharper edges of the enamel along the sides of the exposed track may be secured,

Fig. 6 is a transverse view of Fig. 5,

Fig. 7 is an enlar ed transverse sectional view of the expose part of a resistor in which the tube of Figs. 6 and 7 is used,

Fig. 8 is an elevation view of the complete the tube of resistor arranged 'as a rheostat,

Fig. 9 is a plan view of the same, and F-ig.o 10 is a transverse sectional view taken along the line 10-10 of Fig. 8.

`In practicing the method of the invention a form l, such as a tube of refractory 'material, is wound with suitable wire 2, the

ends of which are attached to contact bandsl 3, the ends of which extend outwardly to serve as terminals 4 and are suitably fastened together, as shown in Fig. 2. The

resistance unit thus produced is completely coated with suitable enamel slip 5 in the usual way and then allowed to partially dry in the air.

The unit is then placed in the device shown in Figs. 1 and 2. The device consists of a base 6 provided with standards 7 and 8 at its ends. Standard 7 has an inwardly extending conical centering member 9 adapted to enter one end of the tube 41. A rod 10, provided with a knob 11 is slidably mounted in the other standard 8. To its inner end is attached a similar centering member 12 ,adapted to enter the other end of the tube 1 and be held therein by a spring 13 surrounding the rod between the standard 8 and the centerin member 12. By means of knob 11 the mem er may be retracted to permit the unit to be placed in or removed from the device. To aid in properly positioning the unit in the device, the base is provided witha projection 14 against which one of the terminals 4 may be placed.

A shield 15 is hinged upon a rod 16 extending between the standards 7 and 8 and is provided with a slot 17 of a width corresponding substantially to that of the desired width of the exposed area on the resistor, the sides'of the slot being preferably' bevelled as shown most clearly in 2. In order to revent the slide from coming in contact with the damp enamel slip and sticking to it, adjustable set screws 18 are placed at the outer corners of the shield 15 and rest upon lugs 19 projecting inwardly from the standards 7 and 8.

After the partially dried unit has been placed in the device with the shield lifted, the latter is lowered until the set screws 18 engage the lugs 19, the screws being properly adjusted to hold the shield close to but out of contact with the enamel on the unit.

The unit being in place between the centering members and the shieldn lowered over it, the enamel slip is brushed off by passing a bristle brush along the`slot in the shield. Thel unit is then removed and is ready to be fired. The required number of coats having been applied in accordance with the procedure described above, it will be found that there is a very thin skin of oxide and enamel on the exposed portions of the wire. This may be removed by placing the unit in the same, or in a similar, shield device and rubbing the exposed portion with emery cloth orother suitable abrasive. Very 'little effort is re uired to give a bright surface to the expose wire in the track 20.

The appearance of the resulting resistor is best illustrated in Figs. 3 and 4 wherein the exposed outer surfaces of the wire 2 between the edges of the enamel 5 at the sides of the track 20 are shown as projecting slightly above the enamel which during the brushing operation has remained between the turns of the wire. This enamel after the iiring operation keeps the turns properly spaced and holds them firmly against displacement when a slidable Contact device is used with the wire as Will presently be described.

A further sharpening of the edge of the enamel along the sides of the exposed track may be secured by the use of longitudinal slots in the refractor tube as shown in Figs. 5 and 6. In these gures the tube is indicated at 1 with the two slots 21. The particular shape of these slots is not important, but they are so located that when the wound unit is placed in the device, these slots come ateither edge of the areas which it is proposed to expose by brushing. In other respects the process remains exactly the same. While the use of these slots is beneficial, they are not essential, and commercially satisfactory units can be produced without them. Their action, apparently, is to serve as a small reservoir of fiuid enamel in the furnace and thus prevent the surface tension from pulling the enamel away from the exposed track. The resulting resistor is shown in Fig. 7 which shows how the enamel has flowed into the slots 21.

In the manufacture of resistors of the type described in this specification, it is found wire will expand away from the tube in the ring process and the surface tension of the enamel may pull the turnsY apart in some -places and bunch them together in others,

making an uneven spacing of turns in the finished unit.

The resistor produced as described above is particularly adapted for use as the resistance unit of a rheostat or potentiometer. Such an application of the invention is shown in Figs. 8, 9, and 10. ,The resistor indicated generally by 22 is clamped between two end members 23 by a rod 24 with washers 25 interposed between the resistor and the members to take care of relative expansion between the metal of the rod and the material of the tube. The end members serve as supports for the device andalso carry between them aI rod 26, preferably of rectangular cross section and insulated from the members by bushings 27. The rod is -held in place by nuts 28 on its threaded ends, one of the ends being provided with binding post elements 29. The terminals 4 of the resistor are also provided with -binding posts 30 for connections to an external circuit.

Slidably mounted on the rod 26 is a contact device 31 of the following construction:

A tubular member 32, preferably of rectangular shape, is vplaced over the rod 26, and a spring 33 is placed between the bottom of the tubular member and the lower side of the rod, the free end of the spring bearing l a upon the exposed track on the resistor. A

metallic block 34 is placed between the top of the tube and the upper side of the rod. F or the purpose of adjusting the Contact device a member 34 of insulating material is placed over the tube and the block and is connected to these elements by a screw 35 having its head set in a depression in the member to avoid danger of contact with the fingers of the operator when the device 1s moved along the rod to adjust its position relatively to the exposed track.

While certain preferred embodiments of lthe invention have been disclosed, it will be understood that it `may be emplo ed in the production of resistors of various orms having exposed areas of various shapes and that various changes in the details of construction of the devices shown herein as well as modifi-4 cations of the method described herein may be made without departing from the principles of invention as defined in the appended claims.

I claim:

. l. The method of producing a resistor which consists in winding a wire upon a V refractory form, completely coating the wire and form with vitreous enamel slip, brushing olf the slip over an area to expose the wlre and subjecting the slip to high temperature to lire the same.v

2. The method of vproducing a, resistor which consists in winding a wire upon a refractory form, completely coating the wire andform with vitreous enamel'slip, allowing the slip to partially dry, brushing oil' the partially dried slip over an area to expose the wire and subjecting the slip to high temperature to fire the same.

3. The method of producing a resistor which consists in windinga wire upon a refractory form, under high tension, completely coating the wire and form with vitreous enamel slip, brushing oiii' the slip over an area to expose the wire and sub]ecting the slip to high temperature to lire thesame.

4. The method of producing a resistor which consists in winding a wire upon a refractory form, completely coating the wire and form with vitreous enamel slip, brushing off the slip from the outer portions of the turns ofwire over an area to expose the wire while leaving some of the slip between the lturns and subjecting the slip to high temperature to fire the same.

5. The method of reducing a resistor which consists in lwinding a wire upon a refractory form, completely coating the wire and form with vitreous enamel slip, brushing oft' the slip from a longitudinal track to expose the wire and subjecting the slip to high temperature to fire the same.

In testimony whereof I aliix m signature.

' GEORGE B.- ROUSE.

Images