. The Ontario high school physics. lamps in anight of 10 hours ? 2. The difference in potential between a trolley wire and the rail whichcarries the return circuit is 5U0 volts, and the motor of a car takes anaverage current of 25 amperes. How much work is done each hour inthe circuit joining the trolley wire and the rail 1 3. Find the horse power necessary to run an electric light installationtaking 125 amperes at 110 volts. 4. The resistance of the filament of an incandescent lamp is 200 ohmsand it carries a current of 6 amperes. Find the amount of heat (incalories) developed in this filamen
. The Ontario high school physics. lamps in anight of 10 hours ? 2. The difference in potential between a trolley wire and the rail whichcarries the return circuit is 5U0 volts, and the motor of a car takes anaverage current of 25 amperes. How much work is done each hour inthe circuit joining the trolley wire and the rail 1 3. Find the horse power necessary to run an electric light installationtaking 125 amperes at 110 volts. 4. The resistance of the filament of an incandescent lamp is 200 ohmsand it carries a current of 6 amperes. Find the amount of heat (incalories) developed in this filament per minute. 5. A 25-candle power tungsten lamp, when used in a 25-volt circuittakes one ampere of current. Find its efficiency. 6. The potential difference between the wires entering a house is 104volts, and an average current of 8 amperes flows through them for 4 hoursper day. How many watt-hours of energy nnist the householder pay forin a month of 30 days ? Find the cost at 8 cents per kilowatt-hour.(1 kilowatt = 1000 watts.). 468 ELECTRICAL MEASUREMENTS . 554. Resistance Boxes. The standavd resistance was definedin § It is obvious that for (he purpose of comparingresistances it would be incon\enient to use mercury columnsin ordinary experiments. In practical work resistance coilsare used for this purpose. Lengths of wiie of known resist-ance are wound on bobbins and connected in sets in resistanceboxes. Fig. 559 shows the common method of joining thecoils. A current in passing from A toB meets with practically no resistancefrom the heavy metallic bar when allthe plugs are inserted. To introduce agiven resistance, tlie plug short-circuitingFia. ^nn^tioM in a the proper coil is removed and the currentresistance box. j^ ^^^.^^|^ ^^ travcrse the resistance wire. For convenience in calculation the coils are usually groupedvery much as weights are arranged in boxes. For example,a set of coils of 1, 2, 2, 5, 10, 10, 20, 50, 100, 100, 200, 500ohms may be combined to
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