The magnet ' is wound to a resistance of amperes with accumulators).
The instrument therefore does not begin to read from zero current, but from some higher limit which, generally speaking, is about one-tenth of the maximum, so that an ammeter reading up to io amperes will not give much visible indication below i ampere.
Very convenient and accurate instruments based on the above principles have been devised by Lord Kelvin, and a large variety of these ampere balances, as they are called, suitable for measuring currents from a fraction of an ampere up to many thousands of amperes, have been constructed by that illustrious inventor.
Thus the centi-ampere balance ranges from i to ioo centi-amperes, the deci-ampere balance from i to ioo deci-amperes, the ampere balance from i to ioo amperes, the deka-ampere balance from i to ioo amperes, the hecto-ampere balance from 6 to 600 amperes, and the kilo-ampere balance from ioo to 2500 amperes.
In these circumstances the current is known to have a fixed value in amperes determined by the weight attached to the instrument.