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. [Illustration: _Fig. 117. Arc-Lighting Circuit._] 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. [Illustration: _Fig 118. Interrupted Conductor._] 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. The result would be that the small connection B would heat up, and, finally, be fused. While the large part of the wire would carry 500 amperes, the small wire could not possibly carry more than, say, 10 amperes. Now these little wires are the filaments in an electric bulb, and originally the attempt was made to have them so connected up that they could be illuminated by a single wire, as with the arc system above explained, one following the other as shown in Fig. 117. [Illustration: _Fig. 119. Incandescent Circuit._] It was discovered, however, that the addition of each successive lamp, so wired, would not give light in proportion to the addition, but at only about one-fourth the illumination, and such a course would, therefore, make electric lighting enormously expensive. This knowledge resulted in an entirely new system of wiring up the lamps in a circuit. This is explained in Fig. 119. In this figure A represents the dynamo, B, B the brushes, C, D the two line wires, E the lamps, and F the short-circuiting wires between the two main conductors C, D. It will be observed that the wires C, D are larger than the cross wires F. The object is to show that the main wires might carry a very heavy amperage, while the small cross wires F require only a few amperes.