_=352. Measurement of Current Strength.=_ The _galvanoscope_ previously described simply shows the presence of a current, or whether one current is larger or smaller than another. When the degree-card is used to get the relative deflections, the instrument may be called a _galvanometer_. _=The Tangent Galvanometer=_ is made on the same general idea as our galvanoscope, the diameter of the coil being twenty times, or more, the length of the needle. In these the strengths of the two currents compared are proportional to the tangents of the angles of deflection produced. (See Elementary Electrical Examples.) There are several varieties of galvanometers, each designed for its special work. They are often calibrated or standardized so that the amperes of current passing through them can be read off directly from the scale. _=353. The Ammeter=_ is really a galvanometer from which may be read directly the strength of a current. The coil has a low resistance so that it will not greatly reduce the strength of the current to be tested. _=The Voltameter=_ measures the strength of a current by chemical means. _=354. Unit of Quantity; The Coulomb.=_ A current having a strength of 1 ampere will do more chemical work by flowing one hour than it can do in 1 second. In speaking of the _quantity_ of electricity we introduce the element of _time_. The unit of quantity is called the _coulomb_, just as a cubic foot of water may be taken as a unit of quantity for water. A coulomb is the quantity of electricity given, in one second, by a current having a strength of 1 ampere. Coulombs are found by multiplying amperes by seconds; thus, a current of 5 amperes will give 20 coulombs in 4 seconds. _=355. Electrical Horse-power; The Watt.=_ The electric current has power to do work, and we speak of the horse-power of an electric motor in the same way as for a steam-engine. A current with the strength of 1 ampere and an E. M. F. of 1 volt has a unit of power called the watt. 746 watts make an electrical horse-power. Watts = amperes × volts. Watts ÷ 746 = the number of horse-power. (See Transformers, also Elementary Electrical Examples.) _=356. Ohm's Law.=_ It was first shown by Ohm that the strength of a current is equal to its E. M. F. divided by the resistance in the circuit; that is, Strength of current (amperes) = E. M. F. (volts). / resistance (ohms). If we let C stand for the strength in amperes, E for the E. M. F. in volts, and R for the resistance in ohms, we have the short formula, easily remembered, C = E/R _=357. An Ampere=_ would be produced by a current of 1 volt pushing its way through a resistance of 1 ohm. Knowing any two of the three, C, E, or, R, the other may be found. The resistance, R, it must be remembered, is the total resistance in the circuit, and is composed of the total internal and external resistances. (See Elementary Electrical Examples.)