The other end of the wire is then brought upwardly through the
interior of the coil and secured in like manner to the other screw (D).

Caution must be used to prevent the different coils or turns from
touching each other. When completed, the coil may be immersed in water,
the current turned on, and left so until the water is sufficiently
heated.

[Illustration: _Figs. 97-98._ RESISTANCE DEVICE]

HOW TO ARRANGE FOR QUANTITY OF CURRENT USED.--It is difficult to
determine just the proper length the coil should be, or the sizes of the
wire, unless you know what kind of current you have. You may, however,
rig up your own apparatus for the purpose of making it fit your heater,
by preparing a base of wood (A) 8 inches long, 3 inches wide and 1 inch
thick. On this mount four electric lamp sockets (B). Then connect the
inlet wire (C) by means of short pieces of wire (D) with all the sockets
on one side. The outlet wire (E) should then be connected up with the
other sides of the sockets by the short wires (F). If, now, we have one
16-candlepower lamp in one of the sockets, there is a half ampere going
through the wires (C, F). If there are two lamps on the board you will
have 1 ampere, and so on. By this means you may readily determine how
much current you are using and it will also afford you a means of
finding out whether you have too much or too little wire in your coil to
do the work.

[Illustration: _Fig. 99._ PLAN VIEW OF ELECTRIC IRON]

AN ELECTRIC IRON.--An electric iron is made in the same way. The upper
side of a flatiron has a circular or oval depression (A) cast therein,
and a spool of slate (B) is made so it will fit into the depression and
the high resistance wire (C) is wound around this spool, and insulating
material, such as asbestos, must be used to pack around it. Centrally,
the slate spool has an upwardly projecting circular extension (D) which
passes through the cap or cover (E) of the iron. The wires of the
resistance coil are then brought through this circular extension and
are connected up with the source of electrical supply. Wires are now
sold for this purpose, which are adapted to withstand an intense heat.

[Illustration: _Fig. 100._ SECTION OF ELECTRIC IRON]

The foregoing example of the use of the current, through resistance
wires, has a very wide application, and any boy, with these examples
before him, can readily make these devices.

THERMO ELECTRICITY.--It has long been the dream of sci