Electricity may be defined as matter in motion along definite lines through material capable of having its particles moved by this force. When the particles yield readily to the force brought to bear upon them, their temperature is very slightly altered; but when they resist electric force, their temperature rises rapidly. When many lines of force are brought to bear on a small number of particles, the great strain on them causes their temperature to rise considerably and, in some cases, entirely overcomes their bonds of cohesion. This is the case when a large number of lines are forced through a thin carbon rod. The particles of carbon resist electric force and get warm, then hot, then red-hot, then white-hot, then incandescent, and finally break away under the great strain. When two such rods are placed end to end to form a conductor of electric current, their particles at the ends are imperfectly connected, and their consequent resistance is higher; they therefore soon become white-hot and break away from the solid mass, but at the same time they heat the air around them, and this hot air also conducts the current Small particles of carbon, glowing at a white heat, are carried by this current of heated air from one carbon rod to the other, and others are turned into incandescent vapor in the space between them, to form an arched bridge of light known as the electric arc light.

To produce this arc the electric current must have volume sufficient to make the carbon particles white-hot, and sufficient electro-motive force to carry them over the space between the tips of two carbon rods. The volume of current required to incandesce carbon rods at their tips in an arc lamp may be put down as 5 amperes for each 1/4 in. in diameter, while the e. m. f. is a constant of from 45 to 50 volts for all ordinary arcs these being formed when the carbon rods are 1-12 in. apart. The light given by an electric arc lamp maybe put down as 500 candle power for a 5-ampere lamp. If, therefore, two carbon rods, each 1/4 in. in diameter, are so placed together that their two ends firmly touch each other, and an electric current of 5 amperes and 50 volts is made to pass through the carbons, the point of contact will first get red-hot, then white-hot, then glow with much brilliance, while the carbon particles burn away, until the gap between the ends of the carbon becomes 1-12 in. wide. A bridge of burning carbon in the form of an arc will then emit a brilliant light for a few moments, while the arc lengthens to 1/8 in. and then expires, unless the carbon rods are moved towards each other and the arc gap maintained.

An arc lamp, therefore, is a small machine made to hold the carbons and move them towards each other as they burn away, so as to maintain the right space between them to form the arc. But it must do more than this. It must be so constructed as to separate the carbon rods when they are hot enough to give the necessary light, and thus " strike the arc". Numerous devices have been invented to attain this end, and some wonderful pieces of clockwork mechanism, combined with electro-magnetic apparatus, have been sold as arc lamps. One of the most simple forms, embracing the leading principles of construction in the lamps, is shown in the accompanying illustration, where, fixed by its upper part to a platform a of wood or of sheet-ebonite, is a hollow brass bobbin, b. A soft iron rod.c, nearly fills the hollow space in this bobbin, and is made to slide in it freely. Tbe lower part of the rod is enlarged to form a socket to hold the upper carbon rod, also a small milled terminal screw for attachment to the flexible conductor shown in the sketch. The bobbin is wound with two coils of wire. A thick wire coil, d capable of carrying the full current consumed in the lamp is connected in series with the carbon-holder (this coil makes only a few turns round the bobbin), and a thin wire coil of several ohms resistance is wound over this, as shown by the thin line. This coil is connected in shunt with the main circuit to the two terminals, e, on the platform. The lower carbon is held in a suitable socket attached to the bracket, f, the upper part of which is connected to one of the terminals above.

When this lamp is unconnected with an electric current, the two carbons rest upon each other and are kept in close contact by the weight of the iron rod. But when a sufficiently strong electric current is connected to the two terminals, e, it traverses the thick coil, d, and converts the air space within the bobbin, b, into an electro-magnet, called a solenoid (which has the property of sucking the iron core, c, into itself); the carbon-holder is therefore drawn up, the carbons are separated, and the arc is struck.

If the current is too strong, the upward pull of the solenoid will jerk the carbons violently apart, and, as this will break the circuit, they will as suddenly fall together again, with a resulting pumping action and unsteady, flashing light. To counteract this fault, the bobbin is also wound with many turns of a long, thin wire in the opposite direction, and this wire is connected in shunt across the terminals. A part of the current therefore traverses the fine coil, and induces a contrary magnetism in the solenoid, which serves as a brake on the otherwise violent upward pull of the main current. When the turns of the wire in the main coil are properly proportioned to the turns of wire in the fine coil, the two coils so balance each other as to maintain the gap between the ends of the carbons at its proper length to produce a steady arc light. In an arc lamp of this design, the magnetic solenoid acts against gravity to pull the top carbon up from contact with the bottom one. As it does this the arc is first formed, then lengthened, and with the lengthening a higher resistance to the current is created, with the result that less current passes through the main coil and the magnetism of the solenoid is weakened. The carbon therefore drops a little, and the arc is shortened; but as the resistance of the main circuit increases, more current goes through the fine coil and checks the downward course, as it checked the upward course, and thus the balance is restored when the normal arc is formed.

In a good lamp, well designed and adjusted and kept clean, these intervals of up and down movement are so short as to make the feeding of the carbons appear continuous. The proportionate resistance of the two coils is usually 1 to 100; that is, fhe series coil as 1 ohm, and the shunt coil 100 ohms. The gauge of copper wire employed in the series must be large enough to safely carry the current required to maintain the arc light. The flexible wire cord connecting this coil to the upper carbon must have the same capacity. If the current to be carried by the series coil is 5 amperes, the wire in this coil should be No. 14 s. w. g. copper, cotton-covered and paraflned; and the flexible wire cord should be 100-36 silk braided.

The illustration given is only intended to show the principles of construction in one of the most simple designs of arc lamps. An arc lamp embodying these principles can be constructed and designed by almost anyone following the hints here given. First decide upon the candle-power of the lamp, and choose a suitable pair of carbon rods. The sizes given by makers run from 8 mm. to 30 mm. and vary in price from l 1/2c to 12c per foot-length. The smaller diameters are for lamps of low candle-power, taking a small current. The rods are made solid for the lower carbon, and cored for the upper carbon. On referring to the illustration, it will be seen that the lower carbon rod is pointed and is marked -, indicating that is the negative carbon and most be connected to the negative pole. It is a solid rod, and as it does not burn away so fast as the upper carbon, it may be made shorter. The uppercar-bon has a slightly hollowed end and is marked + to show that it is the positive and so must be connected to the positive pole. The current will therefore pass from the upper carbon to the lower, and frit off more particles from the upper rod than it does from the lower one. As it burns with a hollow center - called a crater - it is made with a soft core. These facts must be noted when buying the carbon rods, the length of which must be determined by the number of hours they are required to burn, the approximate consumption being from 1 3/4 to 3 in. per hour for the two rods; the proportion for each rod being 2 to 1, that is, 2 in 3 for the positive, and 1 in 3 for the negative carbon.

When the size and length of the carbon rods have been determined, all the dimensions of the other parts of the lamp can be adapted to them. The diameter of the carbon holders must be adapted to that of the carbon rods. The length of the upper carbon holder with its iron core must also be adapted to the carbon, and the bobbin to the solenoid coil to the iron core working in it. After this, the dimensions of the frame and platform can easily be determined. If it is desired to enclose the upper part of the lamp in a brass case, this can be added, together with any other conveniences, after the lamp is made. Connection is secured between the lamp and the source of current by means of flexible wire cables.

The foregoing principles of construction have a modified application in some of the best lamps. In the Brockie-Pell lamp, two solenoids, side by side, regulate the arc by means of a lever and brake action. In the Brush lamp there are two sets of cartons, worked with two solenoids side by side, and regulated by a triangular clutch between them. In the Angold lamps, two solenoids actuate a lever and ratchet device, and a brake to regulate one set of carbons.