the same
gauge, or 93 feet. This resistance wire can be wound in spirals and
made to occupy a very small space. As long as it is connected in
circuit, the energy of the dynamo otherwise consumed as light would be
wasted as heat. This heat could be utilized in the hot water boiler or
stove when the lights were turned off.

In actual practice, however, the resistance necessary to keep the
dynamo up to full load permanently, would not be furnished by one set
of resistance coils. Each lamp circuit would have a set of resistance
coils of its own. A double-throw switch would turn off the lamps and
turn on the resistance coils, or _vice versa_.

Let us say a lamp circuit consisted of 6 carbon lamps, of 16
candlepower each. It would consume 6 x 1/2 ampere, or 3 amperes of
current, and interpose a resistance of 36.6 ohms--say 37 ohms. Three
amperes would require a wire of at least 1,500 circular mills in area
for safety. This corresponds to a No. 18 wire. A No. 18 copper wire
interposes a resistance of one ohm, for each 156.5 feet length. For 37
ohms, 5,790 feet would be required, for copper wire, which of course
would be impractical. Dividing by 30 gives 193 feet for 30% German
silver wire; and dividing by 60 gives 96 feet of nichrome wire of the
same gauge.

It is simple to figure each circuit in this way and to construct
resistance units for each switch. Since the resistance units develop
considerable heat, they must be enclosed and protected.

_A Home-made Stove or Radiator_

While we are on the subject of resistance coils it might be well here
to describe how to make stoves for cooking, and radiators for heating
the house, at small expense. These stoves consist merely of
resistances which turn hot--a dull red--when the current is turned on.
Iron wire, German silver wire, or the various trade brands of
resistance wire, of which nichrome, calido, and manganin are samples,
can be used. In buying this wire, procure the table of resistance and
carrying capacity from the manufacturers. From this table you can make
your own radiators to keep the house warm in winter. Iron wire has the
disadvantage of oxidizing when heated to redness, so that it goes to
pieces after prolonged use. It is cheap, however, and much used for
resistance in electrical work.

Let us say we wish to heat a bathroom, a room 6 x 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 r