1. The longer the wire the more resistance it offers to the electric current. 2. The smaller the diameter of the wire the more resistance it offers. 3. Some materials offer more resistance than others, for example, iron about six times as much as copper and German silver about twelve times as much as copper. 4. The common metals offer more resistance when hot than when cold, about double the resistance when heated to five hundred degrees. It is the reverse with carbon, which offers more resistance when cold than when hot. The carbon filament lamp offers about double the resistance when cold as when lighted to full brilliancy. 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. [Illustration: Fig. 84] [Illustration: 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. (110 volts)/(330 + 220 ohms) = .2 ampere