Arc lights are produced by what is called the series current. This means that the lamps are all connected in a single line. This is illustrated by reference to Fig. [117], in which A represents the wire from the dynamo, and B, C the two electrodes, showing the current passing through from one lamp to the next.
A high voltage is necessary in order to cause the current to leap across the gap made by the separation of the electrodes
The Incandescent System.—This method is entirely different from the arc system. It has been stated that certain metals conduct electricity with greater facility than others, and some have higher resistance than others. If a certain amount of electricity is forced through some metals, they will become heated. This is true, also, if metals, which, ordinarily, will conduct a current freely, are made up into such small conductors that it is difficult for the current to pass.
In the arc method high voltage is essential; in the incandescent plan, current is the important consideration. In the arc, the light is produced by virtue of the break in the line of the conductor; in the incandescent, the system is closed at all times.
Supposing we have a wire A, a quarter of an inch in diameter, carrying a current of, say, 500 amperes, and at any point in the circuit the wire is made very small, as shown at B, in Fig. [118], it is obvious that the small wire would not be large enough to carry the current.