limited only by the consideration of their cost. To sum this up, the whole pressure of the current may be regarded as being lost in passing through the lamps, motors, and other apparatus in a well-designed installation.
The larger the section of a wire, the less resistance does it offer to the passage of the current. Therefore one wire double the diameter of another will offer four times less resistance. On the other hand, if the current passing through any wire is doubled, the loss of pressure in travelling a given distance will be four times greater than before; three times the current, nine times: i.e., the waste increases as the square of the current. In electric lighting, the waste invariably consists in producing heat at places in the circuit where heat is not required; and if this is generated in too great a degree, by reason of the mains offering too much resistance,—that is, being too small in section,—a fire may result. To avoid the possibility of such an accident, the current is made to pass in its course, at suitable points, through short pieces of metal far more fusible in nature than the material of which the conductors are made. The cables and wires are usually of copper. The fuses, as a rule, consist of tin wire, and are generally called "safety- junctions." Then, if the current rises beyond a certain limit from any cause, the safety-junctions melt and cut the circuit before any damage is done.
The only apparatus in connection with an electric-light installation that need be considered here are the arc lamp, the incandescent lamp, the motor, the switches, and the instruments for indicating the quantity and pressure of the current.
In the arc lamp the current passes through two carbon rods, which are separated from each other by a very short distance. In order that the current shall leap this interval, the rods are made to touch each other, and then they are separated: a flame, consisting of heated gases, passes between these carhon rods, which flame must not be mistaken for visible electricity. The powerful light is produced by the intense heat to which the ends of the rods are raised. Suitable apparatus is connected with these carbons, in order that they may be fed as they burn away. Otherwise the distance between them will increase, and eventually the current will cease to flow. This form of light is termed "arc" light, because the flame resembles in shape an arc or a crescent.
The incandescent or glow lamp consists of a very fine filament of carbon, hermetically sealed in a glass globe from which the air has been exhausted. The ends of the filament reach the outside of this globe by being attached within it to two platinum wires which pass through the glass to the outside, where they are dealt with in some convenient way whereby they may be attached to the circuit. The current consequently enters the filament at one end and leaves it by the other. The filament becomes white hot during the time that the current passes through it, and is not consumed, since it is not in the presence of air. The high resistance of the carbon filament necessitates a great loss of pressure in the current during its passage, and is oonverted into light-giving heat. If the pressure of the current is greater than that for which the lamp was constructed, too much cur-
