Two New Potentiometers

Although the high resistance po-i tentiometers (voltage dividers) used in the control circuits of vacuum tubes and photo-electric cells are modified versions of the rheostats and potentiometers used in the early days of radio for the control of filament, grid, and plate voltages, present-day needs make many improvements desirable. Since many vacuum-tube circuits are calibrated it is, of course, important that resistors for use with them be fairly stable and be as free as possible from vagaries due to changes in contact resistance, skipping, and other ills.

For the past several years General Radio Type 214 and Type 371 Potentiometers have met most all requirements except in high-quality, voice-frequency circuits where special logarithmic volume controls have been designed. It has been felt that some improvement could be made in controls for other circuits and the Type 314 and Type 471 Potentiometers about to be described are the result of work in this direction.

The Type 314 and Type 471 Poten tiometers are similar in many respects to their predecessors, the Type 214 and Type 371 Potentiometers. The general method of winding the resistance units is similar, except that it has been found possible to use a thinner winding form which reduces the inductance somewhat.

The total resistance of these units has in the past been severely limited in the high resistance ranges by the suiallness of the wire. Even with the use of wire of the greatest practicable resistivity, it has been found almost impossible logo to values much greater than 50,000 ohms even on the large size form used for the Type 371 Potentiometer. The small size wire causes an appreciable amount of spoilage during manufacture and even the finished product is liable to damage if handled roughly. The new units are provided with a bakelite strip which surrounds the form carrying the resistance wire. This acts as a mechanical protection and practically eliminates the possibility of damage to it during its installation in experimental equipment.

Type 311 Potentiometers. The fine wire used in winding the new potentiometers is protected from accidental damage hy a tliin strip of linen bakelite

Type 311 Potentiometers. The fine wire used in winding the new potentiometers is protected from accidental damage hy a tliin strip of linen bakelite

The contact arm, bakelite shaft, and large knob used in the new Type 314 and Type 471 Potentiometers

Another limitation due to the small wire size used in earlier designs was the possibility of getting imperfect contact with the slider used. The new units use a new type of slider having four fingers which bear lightly on the wire. The pressure needed to secure good contact and, therefore, the amount of wear which results is considerably reduced by this method. The four con

tact members also serve to reduce the contact resistance and render it more nearly constant. Thus the stability of circuits containing these units is materially improved.

Since high resistance units are likely to be used in vacuum-tube circuits of high impedance, the question of stray capacitances introduced into the system by the operator's hand on the control knob is often of importance. If an associated amplifier happens to have a substantial gain, it may happen that the coupling to the hand may be sufficient to introduce an appreciable amount of 60-cycle hum picked up from power-supply wiring in the vicinity. This effect has been practically eliminated in the new design by the use of an insulating bakelite shaft. This shaft is larger than the metal shaft used on earlier models, so that nothing has been sacrificed in the way of mechanical ruggedness by the change.

An added feature is the use of a larger knob and a non-metallic pointer. This knob is much easier to handle than the smaller one and its use is therefore worth while. The pointer has a dull while finish which eliminates the ob

Type 471 Potentiometers. Contact resistance, wear, and skipping are reduced by the new four-wiper contact arm jection to confusing reflections which were sometimes encountered when metal pointers were used in unfavorable light.

Most of these features can be seen by careful examination of the accompanying photographs. It will be noticed, of course, that the Type 314 and Type 471 Potentiometers are practically identical in design features. The difference is merely a matter of size of the form upon which the resistance wire is wound. This is the same difference that describes the old Type 214 and Type 371 Potentiometers.

The following table lists the resistance, current carrying capacity, code word, and price of each of the new units normally carried in stock.

Type 314 Potentiometers

Resistance

( ">i rren t

( .ode Word

Price

200 oil m h 600 ,, 2000 ,, 6000 ,, 20.(MM)

165 niilliamperes 95 52 30 16

enate

ENDOW

enemy

KN.IOY

enrol

S4.00 4.00 4.00 4.00 4.00

Type 471 Potentiometers

Resistance

C'il r rent

('ode Word

Price

50,000 ohms 100,000 „ 200.000 ,,

14.7 milliamperea 10.4

7.3

erode erupt ksker

$6.00 6.00 6.00

TYPE 214 POTENTIOMETERS

Type 214 Rheostats and Potentiometers are now supplied as potentiometers only. In other words, each unit has three terminals-—-one con nected to the sliding contact and the other two to either end of the resistor. This change is a simplification that will benefit the user, because it will make it unnecessary for him to make any changes in a unit when the direction of rotation is to be reversed.

Prices on two sizes of these units have been reduced, and all orders placed since February 8 have had the benefit of the reduction.

The following table shows the total resistance, maximum current, and prices for all of the Type 214 Potentiometers regularly carried in stock (for Type 214-13, Table Mounting as well as for Type 214-A, Panel Mounting). Only panel mounting models are regularly carried in stock, however.

1'riee

liesistanee

Current

Old

New

0.75 ohm

4 amperes

$1.50

«1.50

2 ohms

2.5 ,,

1.50

1.50

7 ,,

1.3

1.50

1.50

20

0.75 ampere

1.50

1.50

50

500 milliamperes

1.50

1.50

1O0

350

1.50

1.50

200

250

1.50

1.50

400 „

175

1.50

1.50

1000

110

1.75

1.50

2500 „

70

2.00

1.50

Type 371 POTENTIOMETERS

Effective February 8 the prices of all stock sizes of Type 371 Potentiometers wTere reduced. These are listed on pages 28 and 126 of Catalog F and on page 12 of Bulletin 933. The linear models (Type 371-A) are reduced to $4.00 and the tapered model (Type 371-T) is reduced to $5.00.

THE GENERAL RADIO COMPANY mails the Experimenter, without charge, each month to engineers, scientists, and others interested in commun-ication-frequency measurement and control problems. Please send requests for subscriptions and address-change notices to the

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