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The instrument is " tuned” to have a natural period of vibration equal to that of the current in the circuit under test, and its sensitivity is dependent on the accuracy of tuning. It does not respond to other frequencies, and if other than a sine wave is used, it only takes account of the frequency to which it is tuned, so that the circuit may be considered to be traversed by a pure sine wave of this frequency.

It is of the moving coil type, with permanent or electromagnet, and, by altering the tension on the suspension, the instrument can be adjusted to resonate to any frequency, from: 50 to 1,000, without removing the case.

An alternator, tuning fork, vibrating wire, or, for higher frequencies, a microphone hummer may be used as a source of supply.

With an effective resistance of 60 ohms at 100 cycles per second, the sensitivity is about 5 mm, at 1 M. for 1 microampere.

Non-Inductive Decade Resistance Box. (Fig. 38.)

For accurate alternating current measurements the inductance and capacity must be reduced to the lowest possible.

In this box the higher resistances are woven wire gauze, and the lower resistances are of special construction.

FIG. 38.

ANTI-CAPA CITY

NON INDUCTIVE RESISTANCE

R.W.PAUL o LONDON.

4

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