[Illustration: FIG. 1. FIG. 2.]
All measurements are comparative. We measure weights or forces by
comparison with some generally known and accepted unit standard
weights, lengths, areas, and volumes, by comparison with a unit length,
resistance by a standard ohm, and so forth. In the same way currents
could be measured by comparison with a standard current: but this
would be a troublesome process, not only on account of the apparatus
necessary, but also because it would be a matter of some difficulty to
have a standard current always ready for use. In general, measurement
by direct comparison with a standard unit is discarded for the more
indirect method of measuring not the current itself, but its chemical,
mechanical, or magnetic effect. The chemical method is very accurate if
a proper density of current through the surface of the electrodes be
used,[1] but since it requires a considerable time, and, above all, an
absolutely constant current, its use is almost entirely restricted
to laboratory work and to the calibration of other instruments.
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