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126 THE BOY ELECTRICIAN

pointer on the meter being calibrated are then located for
each value.

The voltmeters must be .placed in parallel, or shunt with
each other, and in series with several battery cells. A
switch is arranged so that the voltage of a varying number
of cells may be passed through the meters. To secure
fractional values of a volt, the rheostat is placed in shunt
with the first cell of the battery. Then, by adjusting both
the switch and the rheostat, any voltage within the maxi-
mum range of the battery may be secured.

This means of regulating voltage is a common one, and
of much use in wireless telegraph circuits; as will be ex-
plained later.

When using the meters, it is always necessary that the
ammeter shall be in series and the voltmeter in parallel or
in shunt with the circuit.

Galvanoscopes and Galvanometers

In the first part of Chapter V it was explained that several
turns of wire surrounding a compass-needle would cause
the needle to move and show a deflection if a current of
electricity were sent through the coil.

Such an instrument is called a galvanoscope and may be
used for detecting very feeble currents. A galvanoscope
becomes a galvanometer by providing it with a scale so that
the deflection may be measured. "

A galvanometer is really, in principle, an ammeter the
scale of which has not been calibrated to read in amperes.

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