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but when the current is passed through them they are separated the required distance by means of an electro-magnet whose coils are traversed by the whole current of the lamp. In order to maintain the carbons the proper distance apart, the upper carbon is held by a clutch or other device whose position is controlled by an electro-magnet. The coils of this electro-magnet are a shunt or branch off the main circuit, of high resistance. When the carbons are at their normal distance apart, the current of the shunt circuit is not of sufficient strength to move the clutch from the position in which it prevents the downward motion of the carbon rod, but when the carbon has sufficiently burned away to increase the resistance of the arc to a determined extent, the increased current which is thereby produced through the shunt circuit is then sufficiently strong to release the clutch and permit the carbon to feed downward. In a well regulated lamp this feeding occurs so gradually that it produces no perceptible effect on the steadiness of the light. Arc lamps require a pressure of about 45 volts to maintain the arc across the separated carbons. Formerly arc lamps were always used in series, that is, the current entered the first lamp, passed successively through each following lamp, and from the last lamp, returned to the generator. As each lamp requires 45 volts, and often 100 or 125 lamps are used in series, the voltage of the current will be the sum of the voltages of all the lamps in one series. The lamps as made at the present day require a current of from 8 to 10 ampères. The above description applies to lamps for a direct current. Lamps are also made for alternating currents, but are not so efficient as those described, and are comparatively little used. Another style of lamp which has come into extensive use is adapted to burn on an incandescent light circuit. These circuits are about 100 volts in pressure, and the lamps are run two in series. Each series of two lamps is connected between the conductors leading from the positive and negative pole, or in other words, each pair of lamps is connected in multiple. It is apparent that if in a series circuit any one lamp breaks the circuit no current will pass through any of the lamps. To obviate this trouble each lamp is provided with an automatic cut-out. This consists essentially of an electro-magnet in a shunt circuit, arranged so that if the resistance of the lamp gets too high the current around the magnet increases up to a point at which the magnet moves an armature, which closes a by-path for the main current around the lamp, thus cutting it out of the series. Double carbon lamps were introduced to increase the length of time the lamps would burn without renewing the carbons. In these lamps one pair of carbons are consumed before the other pair come into use. Another lamp designed for longer life of the carbons is known as the incandescent arc lamp. The ordinary arc lamp requires trimming every night, and some of them twice a night. The carbons in the incandescent arc light will burn about a week, or from 80 to 100 hours. This is effected by enclosing the arc in a globe or chamber made as nearly air-tight as possible, consistent with allowing the carbon to feed through the top of it. A small valve opening outwards is connected with the chamber. When the lamp is lighted, the intense heat of the arc expands the air in the chamber and forces 'out a part of it through the valve, which also prevents the admission of outside air to the chamber. The carbons then burn in a rarefied atmosphere, and the leakage of air into the chamber is so slow that the carbons are oxidized but very slowly. This class of lamps has been perfected within two or three years, and its use especially for interior lighting has grown considerably.

Incandescent electric lighting was introduced by Edison in 1880. Previous to ihat date many attempts to “subdivide the electric light,” as the problem was then called, had failed for want of a suitable substance for the filament of the incandescent lamp. Outside of some of the metals which proved failures, carbon the only substance yet known which is a conductor of electricity and will stand a sufficiently high temperature for use as incandescent lamp filaments. The carbon strip or filament is bent into the shape of a horse shoe or loop and placed inside of a glass bulb called the lamp chamber. The lamp chamber is exhausted by means of a mercury pump (See Air Pump) after which it is hermetically sealed. The exhaustion of the bulb should be as perfect as possible, and in order to insure the complete removal of all the air in the bulb, and the occluded gases in the carbon filament, both the bulb and filament are heated to a high temperature during the latter part of the process of exhaustion. Both the exhaustion and the heating of the lamp must be continued up to the moment that it is sealed. The ends of the filament are attached to platinum wires passing through one end of the glass bulb, and are called leading-in wires. These must be of platinum because this metal expands with the heat in nearly the same proportion as glass, and in the case of any other metal the expansion would either crack the glass or allow air to be admitted at the joint. Filaments are prepared from numerous substances, such as silk, hair, wood fibre, cotton, cellulose, &c. These substances require baking at high temperatures in order to reduce them to pure carbon, their other constituents being driven off by the heat. In some processes of making filaments no treatment is required after the baking, as the filament is then turned out perfectly even in cross section throughout its length. Most filaments are not perfectly even in diameter over their whole length, and any unevenness makes either bright or dull spots when lighted. To correct this the filament is burned for a few moments in a hydro-carbon vapor. The spots which burn brightest on the filament are those of smallest diameter, and they have a higher temperature than the rest of the filament. Now carbon is deposited from the hydro-carbon on to the filament when a certain temperature is reached, and the higher the temperature the faster the deposit. The spots of higher temperature will take on the deposit of carbon

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faster than the rest of the filament, and in this way the diameter throughout becomes perfectly even in a few moments.

The size lamp generally used gives 16-candle power and from 10 to 15 of these can be lighted per horse-power. Lamps are made of different efficiencies, the most usual consumption of power being from 3 to 4 watts per candle-power. Incandescent lamps are connected in multiple, that is, each lamp is connected across the two conductors leading from the poles of the generator. In special cases large lamps of high candlepower are connected in series on arc light circuits, and miniature lamps of small candle-power are used several in series on incandescent light circuits. The usual voltage of incandescent lamps is about 100 volts, although some 50-volt and a few 200 volt lamps are in use.

ELECTRIC LOOM. See WEAVING.

ELECTRIC MOTORS, machines for converting electricity into mechanical power, in construction and principle identical with dynamos (see MAGNETO-ELECTRIC MACHINES), and differing only in a few minor points. They are fitted with one or two appliances usually, for stopping and starting them and regulating their speed and power. Being in fact dynamos, they have the same efficiency of about 90 per cent.; that is to say, a motor which is supplied with 100-horse power of electricity will furnish about 90-horse power to the machinery it is driving. The special advantages are that they are small, light, clean, easy to manage, and can receive any amount of power through a small wire led into the building at some convenient out-of-the-way place. Owing to these advantages, power can be obtained and used in many places where it would be out of the question without the aid of electricity. Street cars may be propelled by it, the shafting on the different floors of factories may be driven without connecting belts passing through the floors, and power may be obtained in places where the amount needed is so small that the expense of a steam-engine and attendant would be out of proportion and too great. Among the machines frequently requiring power in such places may be mentioned small elevators, ventilating fans, sewing-machines, and all kinds of machinery. Current for driving motors is obtained either from galvanic batteries, in the case of very small machines, or from the dynamo circuits from which electric lights are supplied. A vast number of motors are supplied with current from lighting stations during the daytime when but little demand for lighting exists. There are two general methods of supplying the electric current for operating motors, known respectively as the multiple are system, which is the same as that by which incandescent lights are supplied, and the series system, by which arc lights are supplied. See ELECTRIC LIGHTING.

In the multiple arc system the electricity is kept at constant pressure, while the quantity which passes into any machine connected to it depends upon the size of the passage which is offered to the electricity, in the same way that the amount of water which will flow out of a tank depends upon how wide the valve is opened. Motors connected to such circuits regulate themselves by cutting off the current when the speed of the motor has reached the proper limit.

Among motors of this kind are the Edison, Thomson-Houston, C. & C., and others. In the series system a definite amount of current is forced through the wire, and the amount of electric power absorbed is regulated by forcing this definite current to do more or less work as it passes through. Motors operating on such circuits cannot regulate themselves by cutting off the current when sufficient speed is attained. In place of this, a number of devices have been made for the purpose of regulating

motors on these circuits. One arrangement for this purpose is that employed in the Crocker. Wheeler motor. It consists of a gramme ring armature revolving in a stationary field magnet and provided with a governor attached to the shaft within the pulley of the machine. When the speed of the motor reaches the proper limit, the governor draws the armature lengthwise out of the field, thereby weakening the power, and keeping the machine running at constant speed under any variation of the work required of the motor. See the Electric Motor and its Applications, Martin and Wetzler ; Dynamo-Electric Machines, S. P. Thompson.

ELECTRIC ORGAN.. Electricity is used in two ways in operating organs-for pumping and for operating the valves leading to the various sounding tubes in accordance with the manipulations of the key-board. In many places, especially in large churches having complicated organs, a part of the organ tubes are at such distance from the key-board as to preclude the air being admitted to them and shut off quickly as is required in playing, by any mechanical means which can be actuated by the weak force with which the organist depresses the keys. To meet this difficulty, the agency of electricity has been invoked, and now in all large organs the connection from tubes to keyboard is made in this way: The valve admitting the air to the sounding tube is controlled by an electro-magnet. This magnet is connected by a wire to the key-board, and is led to a contact point on the under side of the key corresponding to this tube. The wire is also connected to a suitable battery which supplies the current when this key is depressed, and also supplies the wires leading from all the other keys to the other tubes. When the keys are depressed by the organist the magnets throw open the

air valves, and the corresponding notes are sounded without effort of the operator. The application for pumping air for organs is made by the use of an ordinary electric motor (9.v.), the

motor being connected to an electric light circuit to draw its supply of electricity and geared to the bellows of the organ, so as to operate the latter.

ELECTRIC RAILWAYS. The most extensive application of the transmission of energy by electricity is its use for operating electric railways. In all such roads the power for propelling the cars is supplied by a steam engine or other prime mover at a central station, and is converted by a dynamo into electric energy, which is carried by suitable conductors parallel to the car tracks. By means of sliding contacts this current is led to an electric motor upon the car, which converts the electric energy back to mechan. ical energy for propelling the car. The invention of the electric railway is generally credited to Thomas Davenport, a self-taught blacksmith, of Vermont, who constructed several models of electric railways in 1835, some of which are still in existence. Numerous experiments in electric propulsion followed the attempts of Davenport, but no practical commercial system was devised until 1888, just fifty-three years after the original invention. From 1880 to 1888 a number of experimental roads were constructed, but the first successful electric railway in its modern form was opened in Richmond, Va., Feb. 1, 1888. The full-page illustration shows the central station of this road.

On January 1, 1895, the number of electric railways in the United States and Canada was as follows:

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The total number of surface railways in the United States driven by animal power, electricity, and cables is about 950.

The ihree principal parts of an electric railway are: (1) the central station ; (2) the line and track construction ; and (3) the car equipment.

Central Station. The power plant which operates an electric railway is preferably situated as near as possible to the centre of the system of car lines to which it supplies power, and contains a power plant, by means of which the dynamos are driven. The dynamos for railway work, as well as the engines, are very substantially built, owing to the heavy and constant changes in the load, caused by the stopping and starting of cars. In starting the motor car of the average size used on street railways on a level road about twelve to fifteen horse-power is required at the station, which amount, however, may be considerably increased by a bad condition of the track. In starting up, steep grades three or four times as much power may be needed, but this excessive load is only momentary, and the power expended falls rapidly to half or less of that used in

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