For
practical ready use, instruments employing the mechanical or magnetic
effect of the current are alone suitable. We weigh, so to speak, the
current against the force of a magnet, of a spring, or of gravity.
The measurement will be exact if the thing against which we weigh or
counterbalance the current itself retains its original standard value.
Where permanent magnets or springs are used as a balancing force, this
condition of constancy in our weights and measures is not always fully
maintained, and to make matters worse, there is no visible sign by which
a change, should it have occurred, can be readily detected. A spring
may have been overstrained or a steel magnet may have become weakened
without showing the least alteration in outward appearance. To overcome
this difficulty, the obvious remedy is not to use springs or steel
magnets at all, but to substitute for these some other force which
should be either absolutely constant, such as the force of gravity, or
at least should, vary only within narrow limits, and this in accordance
with a definite law.
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