Fig. 84
16. Electric Flasher ([Fig. 84]).—For automatically flashing electric lights. The one which we examined was constructed according to the plan shown in [Fig. 85]. The lighting circuit is brought to the binding posts b and c. A small insulated wire of high resistance connects b and c, being wound around the metal bar a b. The resistance of this wire, when added to that of lamps, permits not more than one fifth of an ampere to pass, and this warms the wire slightly. The bar a b is composed of two strips of metal, brass above and iron below. Heat expands brass more than iron. The result is that when the current is turned on, the bar begins to curve downward until presently it touches the metal base of c. Then the full current required to light the lamps which are in circuit passes. While the circuit is closed through the large metal strips not enough passes through the fine wire to warm it. On cooling, a b curves upward and breaks the connection with c, and now the current begins again to warm up the small wire.
Fig. 85
The flasher that we examined was adapted to operate: one 32-candle-power lamp; or two 16-candle-power lamps; or four 8-candle-power lamps, on a one ampere circuit of 110-volt pressure.
Let us see what would happen if it were connected either with a current of higher voltage or a circuit of more lamps. Suppose we have a 32-candle-power carbon filament lamp in circuit. This requires one ampere to light it. Its resistance when hot is 110 ohms.
(110 volts)/(110 ohms) = 1 ampere.
When cold its resistance is about double or 220 ohms. The German silver wire of the electric flasher offers 330 ohms of resistance, and together they make 550 ohms. Thus the current is cut down to .2 ampere.