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ELECTRIC LAMPS

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ELECTRIC LAMPS

i. When, in any circuit, the electrical pressure remains constant, the current varies inversely as the resistance. 2. When, in any circuit, the current is to remain constant, the electrical pressure must be varied directly as the resistance varies. 3. In any circuit in which the resistance is kept constant, the current varies directly as the electrical pressure.

These facts together constitute Ohm's law, and may be expressed algebraically as follows: When C denotes current, R resistance and E electrical pressure, then

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R

The quantity which we have called electrical pressure is generally called electromotive force. The unit of electromotive force is that which will send a current of one ampere through a resistance of one ohm. This unit of electromotive force is called a volt. Electromotive force is very frequently denoted by E. M. F. The E. M. P. of a daniell cell is very nearly one volt; that of a lead storage-cell is approximately 2.1 volts.

ELECTRICAL UNITS

These three units, the ampere, ohm and volt, whose standard values we have given above, are all logically connected with the three fundamental units, the centimeter, the gram and the second; but the reasoning which leads to the arbitrary definitions given above is too long for this brief sketch. For a full account of this matter see Thomson's Elements of Electricity and Magnetism or Thompson's Elementary Lessons in Electricity ana Magnetism.

HENRY CREW.

Electric Lamps are of two kinds: arc and incandescent. Both depend upon the general principle that the heat developed in any part of a circuit by a constant electric current varies directly as the resistance of that part of the circuit; or, what is the same thing, the heat developed in an electric circuit by a constant electromotive force varies inversely as the resistance. (See ELECTRICITY.)

THE ARC-LAMP

The arc-light was first employed by Sir Humphry Davy, but it became a commercial success only after the modern dynamo had been perfected, say about 1876. An arc-lamp consists generally of two carbon-poles connected with some source of current exerting an electrical pressure of from 30 to 90 volts between the terminals of the carbon. The arc is started by bringing the carbon-poles into contact, and then slowly separating them by a distance of a millimeter or more. The resistance of the vapor thus formed between the poles is so large, that an enormous amount of heat is developed and the ends

of the carbon-poles become white-hot. The positive pole becomes much hotter than the negative pole, so that for purposes of illumination the positive pole is generally placed above the negative one; and when the arc is used in a projecting-lantern the positive pole is always made to point forward, i.e., toward the screen. The temperature of the arc is unknown, but an average of the best measurements indicates that it is about 3,600° C. Owing to the fact that the oxygen of the air is electronegative, the positive pole is consumed about twice as fast as the negative. Hence the arcs used for street-lighting (and therefore automatically fed) are so arranged that the positive carbon is fed in twice as rapidly as the negative. These streetlights are always arranged in series, from twenty to fifty being placed on each circuit, according to voltage. Within the last few years the arc-lamp has been greatly improved by inclosing it in a small glass-hood, which prevents easy access of air and thus greatly lengthens the life of the carbon. This hood at the same time acts as a diffusing screen, and greatly ameliorates the fierceness of the light.

THE  INCANDESCENT LAMP

The incandescent lamp, such as has been used for the last quarter of a century, was perfected by Edison in 1878; and, in its commercial success, naturally followed that of the dynamo. It consists essentially of a slender filament of carbon inclosed in a perfect vacuum. The resistance of this carbon-filament, varying from one or two ohms to several hundred ohms, is very high compared with the resistance in the rest of the circuit, so that nearly all the heat is developed inside the glass-bulb which incloses the carbon-filament. The source of current usually; employed for incandescent lighting either is a dynamo giving a constant voltage or a storage-battery. Lamps suited to this particular voltage are placed in parallel between the "lead" wires, which come from the dynamo or battery. Most incandescent lamps are made with filaments which become white-hot under a pressure of 50 to 55 volts and from 100 to no volts. The amount of candle-power which a lamp gives, when fed at its proper voltage, is determined by the length of the filament, so that for any particular voltage lamps may be had giving generally any candle-power between 8 and 100. In 1899 Nernst, the German chemist, devised an incandescent lamp which requires no vacuum and is said to consume only half the power of the ordinary carbon-filament. The Nernst filament consists of a short rod of magnesium-oxide, which is a poor conductor of electricity when cold, but becomes a good conductor on being heated. To start the light, it is necessary, there-