8.9
Lead 13.2
German Silver 12.2 to 15

From this table it will be seen that, for instance, iron offers six and
a half times the resistance of silver, and that German silver has
fifteen times the resistance of silver.

This table is made up of strands of the different metals of the same
diameters and lengths, so as to obtain their relative values.

SIZES OF CONDUCTORS.--Another thing, however, must be understood. If two
conductors of the same metal, having different diameters, receive the
same current of electricity, the small conductor will offer a greater
resistance than the large conductor, hence will generate more heat. This
can be offset by increasing the diameter of the conductor. The metal
used is, therefore, of importance, on account of the cost involved.

COMPARISON OF METALS.--A conductor of aluminum, say, 10 feet long and of
the same weight as copper, has a diameter two and a quarter times
greater than copper; but as the resistance of aluminum is 50 per cent.
more than that of silver, it will be seen that, weight for weight,
copper is the cheaper, particularly as aluminum costs fully three times
as much as copper.

[Illustration: _Fig. 96._ SIMPLE ELECTRIC HEATER]

The table shows that German silver has the highest resistance. Of
course, there are other metals, like antimony, platinum and the like,
which have still higher resistance. German silver, however, is most
commonly used, although there are various alloys of metal made which
have high resistance and are cheaper.

The principle of all electric heaters is the same, namely, the
resistance of a conductor to the passage of a current, and an
illustration of a water heater will show the elementary principles in
all of these devices.

A SIMPLE ELECTRIC HEATER.--In Fig. 96 the illustration shows a cup or
holder (A) for the wire, made of hard rubber. This may be of such
diameter as to fit upon and form the cover for a glass (B). The rubber
should be 1/2 inch thick. Two holes are bored through the rubber cup,
and through them are screwed two round-headed screws (C, D), each screw
being 1-1/2 inches long, so they will project an inch below the cap.
Each screw should have a small hole in its lower end to receive a pin
(E) which will prevent the resistance wire from slipping off.

The resistance wire (F) is coiled for a suitable length, dependent upon
the current used, one end being fastened by wrapping it around the screw
(C).