350. Strength of Current. The water in a certain tank may be under great pressure, but if it is obliged to pass through long tubes before it can turn a water-wheel, for example, it is evident that the work done will depend not only upon the pressure in the tank, but upon the resistance to be overcome before the water gets to the wheel. The work that the water can do depends upon its rate of flow, and may be used to measure the strength of the current.
The strength of a current of electricity is measured also by the work that it can do, and it depends upon its rate of flow at the point measured. The strength may be determined from its magnetic, heating, or chemical effects.
351. Unit of Current Strength; The Ampere. A current having the strength of 1 ampere, when passed through a solution of silver nitrate under proper conditions, will deposit 0.001118 gramme of silver in one second; if passed through a solution of copper sulphate, copper plates being used for the electrodes, in the solution, 0.0003277 gramme of copper will be deposited in one second. (See Chemical Effects of the Current.) The thousandth of an ampere is called the milliampere. The strength of a current is proportional to the amount of chemical work that it can do per second. (See [§ 357].)
352. Measurement of Current Strength. The galvanoscope previously described simply shows the presence[143] 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[144] 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.