6. If a 16-candle-power lamp requires 0.5 ampere upon a 110-volt circuit what current and voltage will be needed to operate 12 such lamps in parallel? 7. What will it cost to run these lamps 4 hours a night for 30 days at 10 cents per kilowatt hour? 8. If a mercury arc rectifier uses 5 amperes of current at 110 volts alternating current to produce 5 amperes of direct current at 70 volts, what is the efficiency of the rectifier? 9. Compute the heat produced in a 40 watt tungsten lamp in 1 minute. 10. Compute the heat produced in a 60 watt carbon incandescent lamp in 1 hour. (4) THE TELEPHONE =312. The Electric Telephone.=--This is an instrument for reproducing the human voice at a distance by an electric current. The modern electric telephone consists of at least four distinct parts (see Fig. 312); viz., a _transmitter_, an induction coil, an electric battery, and a _receiver_. The first three of these are concerned in sending, or _transmitting_ over the connecting wires a fluctuating electric current, which has been modified by the waves of a human voice. The receiver, is affected by the fluctuating current and reproduces the voice. It will be considered first, in our study. =313. The telephone receiver= was invented in 1876 by Alexander Graham Bell. It consists of a permanent steel magnet, U shaped, with a coil of fine insulated copper wire about each pole. (See Fig. 310.) A disc of thin sheet iron is supported so that its center does not quite touch the poles of the magnet. A hard rubber cap or ear piece with an opening at its center is screwed on so as to hold the iron disc firmly in place. [Illustration: FIG. 309.--The simplest telephone system. It consists of two telephone receivers connected in series on a circuit. It will work, but not satisfactorily.] _The action of the receiver_ may be understood from the following explanation: The electric current sent to the receiver, comes from the secondary coil of the induction coil; it is an alternating current, fluctuating back and forth just in time with the waves of the voice affecting it at the transmitter. This alternating current flows around the coils on the poles of the permanent magnet. When this current flows in one direction, its magnetic field assists the field of the permanent magnet, strengthening it. This stronger magnetic field draws the thin iron disc in front of the poles of the magnet a little closer to them. When the current in the coils flows the other way, its magnetic field weakens the field of the steel magnet, and the disc is drawn back by the force of its own elasticity. Thus the disc of the receiver vibrates with the alternations of the current, and reproduces the same sounds that were spoken into the transmitter. [Illustration: FIG. 310.--A telephone receiver. This receiver has a permanent horseshoe magnet with a coil about each pole.]