408 MAGNETISM AND ELECTRICITY Ohm's law may now be stated as follows: or Current = -Y^ltage. Resistance Amperes = Volts Ohms or, using accepted abbreviations, T /p t\ = E (Electromotive force) R (Resistance) The electromotive force is the maximum potential difference of a cell, that is, the potential difference that exists when no current is being drawn from the cell. The resistance in this formula refers to the total resistance in a circuit, both internal and external. Electrical Power Is Measured in Kilowatt-hours. Electric power is paid for in terms of watt-hours rather than volts or amperes. The watt is equal to the product of the voltage times the number of amperes, just as in proper units, T7I • 1 D v/ Volume I'luid power = rressureX Seconds Volts X Amperes = Watts The kilowatt-hour, the most common electrical-energy unit, is the energy delivered in an hour at the constant rate of one kilowatt. Everyone should be able to make simple calculations which will enable him to determine the relative cost of operation of his various electrical appliances. Supposing that electrical current costs five cents per kilowatt-hour, what would it appliances for one hour? cost to operate the following Appliance
CURRENT ELECTRICITY 499 The label on the washing machine did not list the wattage, but it did list the amperes as 5.4. wattage was calculated as follows: It Since the voltage in this case was 110, the Volts X Amperes = Watts 110 X 5.4 = 594 should be noted, however, that motors are not usually worked to full power. A few such calculations would enable the home-owner to determine what his money is paying for when he pays his electric bill and to tell where to institute economies intelligently. It will be noted that the most power is consumed by household appliances which transform electricity into heat. Heat is produced when an electric current is forced through a resistance. Electric heating devices are usually wound with alloys of nickel and chromium, which offer electrical resistance and at the same time resist destruction by chemical reaction at high temperatures. Most heating elements burn out eventually, i.e., the wires gradually become oxidized by high temperatures or evaporate if in a vacuum. The heat produced by passing a current through a resistance can be demonstrated by short-circuiting a fully charged automobile battery (if you do not care what happens to the battery) with a large iron nail which will become red hot in a very short time. The resistance of metals becomes greater as the temperature rises, because, according to one theory, it becomes increasingly difficult for the electrons to pass through the metal as the agitation of the molecules increases. The electrical-resistance thermometer consists of a coil of wire in a quartz shield which has a very high melting-point. The thermometer may be placed in a furnace and the temperature determined by measuring the increase in its electrical resistance. There Are Two Kinds of Current Electricity Which May Be Run to Appliances through Conductors in Series or Parallel. There are two kinds of current, direct and alternating. Direct current consists of a stream of electrons moving in one direction only and is produced by chemical cells and certain types of generators. Alternating current, produced only by certain types of generators, consists of a stream of electrons which reverses its direction in the conductor many times per second. Two or more conductors may be connected in series or parallel, as shown in Figs. 237 and 238.