In this case the indication of the
pointer must be divided by ten. If a current be sent through the
instrument the wrong way, the needle turns through an angle of 180 deg., and
thus brings the pointer to the side of the dial opposite to where the
scale is. In this position no reading can be taken, and to facilitate
the sending of the current in the right direction a commutator is added,
and the same is so coupled up that when the pointer stands over the
scale the handle on the commutator points to the positive terminal
screw. There is a limit of electromotive force below which the indicator
fails to give reliable readings. For instance, an instrument wound with
100 ohms of copper wire and 900 ohms of German silver can be used for
electromotive forces varying between 300 and 3 volts, but would not be
reliable for measuring less than 3 volts.
For very exact measurements the instrument should be placed north and
south, in the same position in which it was calibrated. Two different
patterns of current indicators are on the table; one with double needles
suspended on a point in the way compass magnets are suspended, the other
with one lozenge shaped needle mounted on an axle and pivoted on jewels,
in every way similar to the needle of the potential indicator first
described.
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