be the hindrance of the twisting string.  As a consequence, the coil swings farther than before; that is, the greater the current, the farther the swing.  Usually a delicate pointer is attached to the movable coil and rotates freely with it, so that the swing of the pointer indicates the relative values of the current.  If the source of the current is a gravity cell, the swing is only two thirds as great as when a dry cell is used, indicating that the dry cell furnishes about 1-1/2 times as much current as a gravity cell.

314.  Ammeters.  A galvanometer does not measure the current, but merely indicates the relative strength of different currents.  But it is desirable at times to measure a current in units.  Instruments for measuring the strength of currents in units are called ammeters, and the common form makes use of a galvanometer.

A current is sent through a movable coil (the field magnet and coil are inclosed in the case) (Fig. 234), and the magnetic field thus developed causes the coil to turn, and the pointer attached to it to move over a scale graduated so that it reads current strengths.  This scale is carefully graduated by the following method.

If two silver rods (Fig. 208) are weighed and placed in a solution of silver nitrate, and current from a single cell is passed through the liquid for a definite time, we find, on weighing the two rods, that one has gained in weight and the other has lost.  If the current is allowed to flow twice as long, the amount of silver lost and gained by the electrodes is doubled; and if twice the current is used, the result is again doubled.

As a result of numerous experiments, it was found that a definite current of electricity will deposit a definite amount of silver in a definite time, and that the amount of silver deposited on an electrode in one second might be used to measure the current of electricity which has flowed through the circuit in one second.

A current is said to be one ampere strong if it will deposit silver on an electrode at the rate of 0.001118 gram per second.

[Illustration:  FIG. 234.—­An ammeter.]

In marking the scale, an ammeter is placed in the circuit of an electrolytic cell and the position of the pointer is marked on the blank card which lies beneath and which is to serve as a scale (Fig. 235).  After the current has flowed for about an hour, the amount of silver which has been deposited is measured.  Knowing the time during which the current has run, and the amount of deposit, the strength of the current in amperes can be calculated.  This number is written opposite the place at which the pointer stood during the experiment.

The scale may be completed by marking the positions of the pointer when other currents of known strength flow through the ammeter.

[Illustration:  FIG. 235.—­Marking the scale of an ammeter.]

All electric plants, whether for heating, lighting, or for machinery, are provided with ammeters, such instruments being as important to an electric plant as the steam gauge is to the boiler.