Let us say we wish to heat a bathroom, a room 6 × 8, and 8 feet high--that is a room containing 384 cubic feet of air space. Allowing 2 watts for each cubic foot, we would require 768 watts of current, or practically 7 amperes at 110 volts. What resistance would be required to limit the current to this amount? Apply Ohm's Law, as before, and we have R equals E divided by C, or R equals 110 divided by 7, which is 15.7 ohms. Forty-two feet of No. 20 German silver wire would emit this amount of heat and limit the current output to 7 amperes. In the Far West, it is quite common, in the outlying district, to find electric radiators made out of iron pipe covered with asbestos, on which the requisite amount of iron wire is wound and made secure. This pipe is mounted in a metal frame. Or the frame may consist of two pipes containing heating elements; and a switch, in this case, is so arranged that either one or two heating elements may be used at one time, according to the weather. An ingenious mechanic can construct such a radiator, experimenting with the aid of an ammeter to ascertain the length of wire required for any given stove. _Regulating Voltage at Switchboards_ The voltage of any given machine may be regulated, within wide limits, by means of the field rheostat on the switchboard. A dynamo with a rated speed of 1,500 revolutions per minute, for 110 volts, will actually attain this voltage at as low as 1,200 r.p.m. if all the regulating resistance be cut out. You can test this fact with your own machine by cutting out the resistance from the shunt field entirely, and starting the machine slowly, increasing its speed gradually, until the voltmeter needle registers 110 volts. Then measure the speed. It will be far below the rated speed of your machine. If, on the other hand, the speed of such a machine runs up to 2,500 or over--that is, an excess of 67%--the voltage would rise proportionally, unless extra resistance was cut in. By cutting in such resistance--by the simple expedient of turning the rheostat handle on the switchboard,--the field coils are so weakened that the voltage is kept at the desired point in spite of the excessive speed of the machine. Excessive speeds are to be avoided, as a rule, because of mechanical strain. But within a wide range, the switchboard rheostat can be used for voltage regulation.