In determining the exact _quantity_ of an electric current, physicists make use of a device called a _coulomb meter_. (See Fig. 247.) This contains a solution of silver nitrate in which are placed two silver plates. The current to be measured is sent through the solution, in at one plate and out at the other. The plate where the current goes _in_, the _anode_, _A_ (Fig. 247), loses in weight since some of the silver is dissolved. The plate where the current goes _out_, the _cathode_, _C_, increases in weight since some of the silver is deposited. By an international agreement, _the intensity of the current which deposits silver at the rate of 0.001118 g. per second is 1 ampere_. This is equal to 4.025 g. per hour. [Illustration: FIG. 247.--A coulomb meter, the anode _A_ is separated from the cathode _C_ by a porous cup.] The _coulomb_ is defined as the quantity of electricity delivered by a current of one ampere during one second. A 40-watt-incandescent lamp takes about 0.4 ampere of current. An arc lamp takes from 6 to 15 amperes. A new dry cell may send 20 amperes through a testing meter. A street car may take from 50 to 100 amperes. =265. The Ammeter.=--The method described above is not used ordinarily for measuring current strengths on account of its inconvenience. The usual device employed is an _ammeter_. This instrument is a _moving-coil galvanometer_. It contains, wound on a light form, a coil of fine copper wire. The form is mounted on jewel bearings between the poles of a strong permanent horseshoe magnet. (See Fig. 248.) As in other moving-coil galvanometers, a soft iron cylinder within the form concentrates the field of the magnet. The form and its coil is held in balance by two spiral springs which also conduct current into and out of the coil. Only a small part of the whole current measured, in some cases only 0.0001 passes through the coil, the larger part of the current passing through a metal wire or strip called a _shunt_[L] (see Fig. 248) connecting the binding posts of the instrument. A fixed fraction of the whole current flows through the coil. Its field crossing the field of the horseshoe magnet, tends to turn until its turning force is balanced by the spiral springs. As the coil turns it moves a pointer attached to it across a scale graduated to indicate the number of amperes in the whole current. [L] A shunt is a conductor or coil connected in parallel with another conductor or circuit. It carries a part of the current. [Illustration: FIG. 248.--Diagram of a commercial ammeter. _S_ is the shunt.] It should be noted that while _all_ of the current measured passed through the ammeter, but a small _part_ goes through the coil.