4. The current for a city telephone system is provided by a storage battery. Why is this better than dry cells at each telephone? 5. An incandescent lamp takes 0.5 ampere at 110 volts. What power is required to operate it? How much _energy_ will it transform in 1 minute? 6. How long would it take for this lamp to use a kilowatt hour of energy? 7. A street car used 100 amperes at 600 volts pressure. What power was delivered to it? Express also in kilowatts and horse-power. 8. An electric toaster takes 5 amperes at 110 volts. If it toasts a slice of bread in 2 minutes, what is the cost at 10 cents a kilowatt hour? 9. An electric flat iron takes 5 amperes at 110 volts. Find the cost of using it for 2 hours at 12 cents a kilowatt hour. 10. A 1/4 kilowatt motor is used to run a washing-machine for 5 hours. What is the expense for this power at 10 cents a kilowatt hour? 11. What is the efficiency of a motor that takes 7390 watts and develops 9 horse-power? 12. How many horse-power are there in a water-fall 212 ft. high over which flows 800 cu. ft. of water per second? Express this power in kilowatts. 13. What horse-power must be applied to a dynamo having an efficiency of go per cent. if it is to light 20 arc lamps in series, each taking 10 amperes at 60 volts? (3) THE HEAT EFFECT OF ELECTRIC CURRENTS =292. The Production of Heat by an Electric Current.=--When no chemical or mechanical work is done by an electric current its energy is employed in overcoming the resistance of the conducting circuit and is transformed into _heat_. This effect has many practical applications and some disadvantages. Many devices employ the heating effect of electric currents, (a) the electric furnace, (b) electric lights, (c) heating coils for street cars, (d) devices about the home, as flat irons, toasters, etc. Sometimes the heat produced by an electric current in the wires of a device such as a transformer is so large in amount that especial means of cooling are employed. Unusually heavy currents have been known to melt the conducting wires of circuits and electrical devices. Hence all circuits for electric power as well as many others that ordinarily carry small currents are protected by _fuses_. An _electric fuse_ is a short piece of wire that will melt and break the circuit if the current exceeds a determined value. The fuse wire is usually enclosed in an incombustible holder. Fuse wire is frequently made of lead or of an alloy of lead and other easily fusible metals. (See Figs. 271 and 272.) [Illustration: FIG. 271.--A type of enclosed fuse.] [Illustration: FIG. 272.--A link fuse (above); plug fuses (below).]