Log-Law Volume Control

Log-Law Volume Control

Completely stable non-wire wound potentiometer.
Available with the following resistance values.

There have been one or two attempts lately to produce a new form of variable high resistance. One of the neatest of these is the A.E.D. volume control. In this component, the resistance element is built up of alternate washers of brass and resistance material. The whole resistance unit is a little over an inch long, and traveling over the edge of this is a little pointer. This pointer, therefore, makes contact with the brass discs and is never actually in contact with the resistance material; since the current can only get from one brass disc to the next through the intervening resistance element, however, the necessary high resistance is obtained. By having a sufficient number of contacts it is possible to get a smooth graduation, and this appears to have been done in the device in question. By the use of an ingenious mechanism, somewhat similar to that of a slow-motion dial, the pointer is geared down about 2 to 1, so that although the actual travel over the resistance unit is little more than an inch the knob makes a full half turn going from maximum to minimum. The resistance washers in the clement are graded so that the variation increases as one goes towards the high-resistance end of the scale.

The grading is approximately logarithmic, as we found out by making several tests at various parts of the scale and plotting the results. Moreover, the resistance range is very wide, since the maximum is in excess of 500,000 ohms, while the minimum is only a few hundred ohms.

Owing to the fact that the area of the resistance washers is nearly half a square inch. the device will carry a larger current than usual. We passed 5 milliamperes through the resistance without obtaining any undue heating. This must be regarded as a very good current-carrying capacity for such a large resistance.

This component is altogether very cleverly thought out and we shall look forward with interest to the production of further instruments from this firm.

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