Fig. 67 shows an _astatic galvanometer_. Fig. 68 shows a _tangent galvanometer_, in which the strength of the current is proportional to the tangent of the angle of deflection. Fig. 69 shows a _D'Arsonval galvanometer_, in which a coil of wire is suspended between the poles of a permanent horseshoe magnet. The lines of force are concentrated by the iron core of the coil. The two thin suspending wires convey the current to the coil. A ray of light is reflected from the small mirror and acts as a pointer as in other forms of reflecting galvanometers. [Illustration: Fig. 68.] =74. The Ammeter=, Fig. 70, is a form of galvanometer in which the strength of a current, in amperes, can be read. In these the strength of current is proportional to the angular deflections. The coils are made with a small resistance, so that the current will not be greatly reduced in strength in passing through them. [Illustration: Fig. 69.] =75. Voltameters= measure the strength of a current by chemical means, the quantity of metal deposited or gas generated being proportional to the time that the current flows and to its strength. In the _water voltameter_, Fig. 71, the hydrogen and oxygen produced in a given time are measured. (See "Study," Chapter XXI.) [Illustration: Fig. 70.] The _copper voltameter_ measures the amount of copper deposited in a given time by the current. Fig. 72 shows one form. The copper cathode is weighed before and after the current flows. The weight of copper deposited and the time taken are used to calculate the current strength. [Illustration: Fig. 71.] =76. Unit of Quantity=; =The Coulomb= is the quantity of electricity given, in _one second_, by a current having a strength of one ampere. Time is an important element in considering the work a current can do. [Illustration: Fig. 72.] =77. Electrical Horse-power=; =The Watt= is the unit of electrical power. A current having the strength of one ampere, and an E. M. F. of one volt has a unit of power. 746 watts make one electrical horse-power. Watts = amperes × volts. Fig. 73 shows a direct reading wattmeter based on the international volt and ampere. They save taking simultaneous ammeter and voltmeter readings, which are otherwise necessary to get the product of volts and amperes, and are also used on alternating current measurements. [Illustration: Fig. 73.] There are also forms of wattmeters, Fig. 74, in which the watts are read from dials like those on an ordinary gas-meter, the records being permanent. Fig. 75 shows a voltmeter V, and ammeter A, so placed in the circuit that readings can be taken. D represents a dynamo. A is placed so that the whole current passes through it, while V is placed between the main wires to measure the difference in potential. The product of the two readings in volts and amperes gives the number of watts. [Illustration: Fig. 74.]