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534 rent will pass through the filament, and it will be destroyed. On the other hand, if the pressure is insufficient, the temperature to which the filament ought to be raised will not be reached, and the light will be far less than it should be under normal conditions. The light given by any lamp diminishes in far greater proportion than the fall in the pressure of the current; and the inverse is true. For instance, a lamp intended to give a certain light with a given pressure of current would give less than half its light with a fall of ten per cent. in pressure. On the other hand, a four-per-cent. increase of pressure above the normal would produce at least double the light intended.

A motor is identical with the dynamo. In the latter case, the armature is revolved and a current produced, but when a current is sent into a dynamo it will be found that the armature revolves: in other words, it becomes a motor. Here is perhaps the simplest way of conveying power that has thus far been discovered. If it is required to turn a lathe or other machine by power at any given place, it will simply be necessary to convey to that place two wires to conduct the current, and then attach a motor. Although heavy, a motor is compact, and a man can, without difficulty, move from place to place, on a properly-designed truck, such a machine up to the size of say ten horse-power. Apart from the rapidity and ease with which motive power may be installed wherever it may be desired, there are also eliminated the dangers, disadvantages, and complications which exist more or less in connection with the forms of motive power hitherto employed.

In order to light and put out a lamp, or to start and stop a motor, the current must be cut: to effect this, simple apparatus are used that break the metallic continuity of the circuit, this being all that is necessary. Such devices are termed switches.

Two instruments are employed to observe what is taking place upon the circuit. The one is termed an ammeter, equivalent to a water-meter, and indicates the quantity of current,—the unit being termed an ampère. The other instrument is called a voltmeter, and registers pressure,—the unit being termed a volt. There are also meters in use, equivalent to a gas-meter, whereby the electrical energy used during any given time is recorded. When current is supplied from a public installation by the unit (which in England is 1000 Watt-hours), the price per unit multiplied by ten gives the equivalent value for gas per thousand cubic feet, light for light, when glow-lamps are used. Thus, electrical energy sold at sevenpence per unit in London is equivalent to gas at six shillings and tenpence per thousand cubic feet.

There are two systems of distribution, one the series and the other the parallel. Of these systems there are many subdivisions of each, but only the simple methods need be referred to.

In the series system the conductor consists of one continuous circle. The lamps and other apparatus which are inserted form a part of this conducting ring. In this method high-pressure currents have to be used, because there is a successive fall of pressure as the current passes through each succeeding lamp or other piece of apparatus. The most