Fig. 27 shows the multiple-series system. Two or more lamps are arranged in series groups, and these groups are connected in parallel between the conductors a b and c f. Manifestly, when one lamp is burned out, all the other lamps in that group will also cease to glow. To obviate this difficulty, automatic devices which close the circuit across the lamps have been used, but have not given complete satisfaction. A diagram of one of these devices is shown in Fig. 28. The lamp is shown at l, an electromagnet in series with the lamp at m, and a is a lever carrying an armature which is attracted by the electromagnet. When the current flows through the lamp, the electromagnet is energized, and the lever a is held up tightly against its stop e. When, however, the lamp is burned out or the filament is destroyed from any cause, the magnet releases its armature, which is pulled by a spring s against the contact f. The circuit is thus closed around b-f-a-d.

Fig. 27.

If there be 200 volts pressure between the conductors, one 200-volt lamp, two 100-volt, or four 50-volt lamps may be connected across.

This system, on account of its adaptability to lamps of different voltages and candlepowers, is occasionally used to light stores or large buildings.