Understanding Terms.—We must now try to ascertain the meaning of some of the terms so frequently used in connection with electricity. If you intended to sell or measure produce or goods of any kind, it would be essential to know how many pints or quarts are contained in a gallon, or in a bushel, or how many inches there are in a yard, and you also ought to know just what the quantity term bushel or the measurement yard means.

Intensity and Quantity.—Electricity, while it has no weight, is capable of being measured by means of its intensity, or by its quantity. Light may be measured or tested by its brilliancy. If one light is of less intensity than another and both of them receive their impulses from the same source, there must be something which interferes with that light which shows the least brilliancy. Electricity can also be interfered with, and this interference is called resistance.

Voltage.—Water may be made to flow with greater or less force, or velocity, through a pipe, the degree of same depending upon the height ofp. 61 the water which supplies the pipe. So with electricity. It may pass over a wire with greater or less force under one condition than another. This force is called voltage. If we have a large pipe, a much greater quantity of water will flow through it than will pass through a small pipe, providing the pressure in each case is alike. This quantity in electricity is called amperage.

In the case of water, a column 1" × 1", 28 inches in height, weighs 1 pound; so that if a pipe 1 inch square draws water from the bottom it flows with a pressure of 1 pound. If the pipe has a measurement of 2 square inches, double the quantity of water will flow therefrom, at the same pressure.

Amperage.—If, on the other hand, we have a pipe 1 inch square, and there is a depth of 56 inches of water in the reservoir, we shall get as much water from the reservoir as though we had a pipe of 2 square inches drawing water from a reservoir which is 28 inches deep.

Meaning of Watts.—It is obvious, therefore, that if we multiply the height of the water in inches with the area of the pipe, we shall obtain a factor which will show how much water is flowing.

Here are two examples:

1. p. 6228 inches = height of the water in the reservoir.
   2 square inches = size of the pipe.
   Multiply 28 × 2 = 56.
2. 56 = height of the water in the reservoir.
   1 square inch = size of the pipe.
   Multiply 56 × 1 = 56.

Thus the two problems are equal.

A Kilowatt.—Now, in electricity, remembering that the height of the water corresponds with voltage in electricity, and the size of the pipe with amperage, if we multiply volts by amperes, or amperes by volts, we get a result which is indicated by the term watts. One thousand of these watts make a kilowatt, and the latter is the standard of measurement by which a dynamo or motor is judged or rated.

Thus, if we have 5 amperes and 110 volts, the result of multiplying them would be 550 watts, or 5 volts and 110 amperes would produce 550 watts.

A Standard of Measurement.—But with all this we must have some standard. A bushel measure is of a certain size, and a foot has a definite length, so in electricity there is a recognized force and quantity which are determined as follows:

The Ampere Standard.—It is necessary, first, to determine what an ampere is. For this purpose a standard solution of nitrate of silver isp. 63 used, and a current of electricity is passed through this solution. In doing so the current deposits silver at the rate of 0.001118 grains per second for each ampere.

The Voltage Standard.—In order to determine the voltage we must know something of resistance. Different metals do not transmit a current with equal ease. The size of a conductor, also, is an important factor in the passage of a current. A large conductor will transmit a current much better than a small conductor. We must therefore have a standard for the ohm, which is the measure of resistance.

The Ohm.—It is calculated in this way: There are several standards, but the one most generally employed is the International Ohm. To determine it, by this system, a column of pure mercury, 106.3 millimeters long and weighing 14.4521 grams, is used. This would make a square tube about 94 inches long, and a little over 1/25 of an inch in diameter. The resistance to a current flow in such a column would be equal to 1 ohm.

Calculating the Voltage.—In order to arrive at the voltage we must use a conductor, which, with a resistance of 1 ohm, will produce 1 ampere. It must be remembered that the volt is the practical unit of electro-motive force

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While it would be difficult for the boy to conduct these experiments in the absence of suitable apparatus, still, it is well to understand thoroughly how and why these standards are made and used.

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