Having given a short account of some of the sources of electricity, let us now proceed to describe some of the practical uses to which it is put, and at the same time describe the operation of the appliances used. Before proceeding further, however, we ought to tell how electricity is measured. We have pounds for weight, feet and inches for lineal measure, and pints, quarts, gallons, pecks and bushels for liquid and dry measure, and we also have ohms, volts, ampÈres and ampÈre-hours for electricity.

When a current of electricity flows through a conductor the conductor resists its flow more or less according to the quality and size of the conductor. Silver and copper are good conductors. Silver is better than copper. Calling silver 100, copper will be only 73. If we have a mile of silver wire and a mile of iron wire and want the iron wire to carry as much electricity as the silver and have the same battery for both, we will have to make the iron wire over seven times as large. That is, the area of a cross-section of the iron wire must be over seven times that of the silver wire. But if we want to keep both wires the same size and still force the same amount of current through each we must increase the pressure of the battery connected with the iron wire. We measure this pressure by a unit called the "volt," named for Volta, the inventor or discoverer of the voltaic battery. The volt is the unit of pressure or electromotive force. (In all these cases a "unit" is a certain amount or quantity—as of resistance, electromotive force, etc.—fixed upon as a standard for measuring other amounts of the same kind.)

The iron wire offers a resistance that is about seven times greater than silver to the passage of the current. To illustrate by water pressure: If we should have two columns of water, and a hole at the bottom of each column, one of them seven times larger than the other, the water would run out much faster from the larger hole if the columns were the same height. Now, if we keep the column with the larger hole at a fixed height a certain amount of water will flow through per second. If we raise the height of the column having the small hole we shall reach a point after a time when there will be as much water flow through the small hole per second as there is flowing through the large hole. This result has been accomplished by increasing the pressure. So, we can accomplish a similar result in passing electricity through an iron wire at the same rate it flows through a silver wire of the same size, by increasing the pressure, or electromotive power; and this is called increasing the voltage.

The quality of the iron wire that prevents the same amount of current from flowing through it as the silver is called its resistance. The unit of resistance, as mentioned in the last chapter, is called the ohm, and the more ohms there are in a wire as compared with another, the more volts we have to put into the battery to get the same current.

The unit for measuring the current is called the "ampÈre," named after the French electrician, A. M. AmpÈre (1789-1836).

Now, to make practical application of these units. The volt is the potential or pressure of one cell of battery called a standard cell, made in a certain way. The electromotive force of one cell of a Daniell battery is about one volt. One ohm is the resistance offered to the passage of a current having one volt pressure by a column of mercury one millimeter in cross-section and 106.3 centimeters in length. Ordinary iron telegraph-wire measures about thirteen ohms to the mile. Now connect our standard cell—one volt—through one ohm resistance and we have a current of one ampÈre. Unit electromotive force (volt) through unit resistance (ohm) gives unit of current (ampÈre). It is not the intention to treat the subject mathematically, but I will give you a simple formula for finding the amount of current if you know the resistance and the voltage. The electromotive force divided by the resistance gives the current. C = E / R or current (ampÈres) equals electromotive force (volts) divided by the resistance (ohms).

But still further: One ampÈre of current having one volt pressure will develop one watt of power, which is equal to 1/746 of a horse-power. (The watt is named in honor of James Watt, the Scottish inventor of the steam-engine—1786-1813). In other words, 746 watts equal one horse-power. By multiplying volts and ampÈres together we get watts.

If we want to carry only a small current for a long distance we do not need to use large cells, but many of them. We increase the pressure or voltage by increasing the number of cells set up in series. If we have a wire of given length and resistance and find we need 100 volts to force the right amount or strength of current through it, and the electromotive force of the cells we are using is one volt each, it will require 100 cells. If we have a battery that has an E. M. F. of two volts to the cell, as the storage-battery has, fifty cells would answer. If we want a very strong current of great volume, so to speak, for electric light or power, and use a galvanic battery, we should have to have cells of large surface and lower resistance both inside and outside the cells.

When dynamos are used they are so constructed that a given number of revolutions per minute will give the right voltage. In fact, the dynamo has to be built for the amount of current that must be delivered through a given resistance. The same holds good for a dynamo as for a galvanic battery. If any one factor is fixed, we must adapt the others to that one in order to get the result we want. There are many other units, but to introduce them here would only confuse the reader. The advanced student is referred to the text-books.

With this much as a preliminary we are prepared to take up the applications of electricity, which to most people will be more interesting than what has gone before.