equire 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.

As it would be a source of continual annoyance to have to run to the
switchboard every time the load of the machine was varied greatly this
plan would not be practical for the isolated plant, unless the
rheostat could be installed,--with a voltmeter--in one's kitche