entists to convert
heat directly into electricity. The present practice is to use a boiler
to generate steam, an engine to provide the motion, and a dynamo to
convert that motion into electricity. The result is that there is loss
in the process of converting the fuel heat into steam; loss to change
the steam into motion, and loss to make electricity out of the motion
of the engine. By using water-power there is less actual loss; but
water-power is not available everywhere.

CONVERTING HEAT DIRECTLY INTO ELECTRICITY.--Heat may be converted
directly into electricity without using a boiler, an engine or a dynamo,
but it has not been successful from a commercial standpoint. It is
interesting, however, to know and understand the subject, and for that
reason it is explained herein.

METALS; ELECTRIC POSITIVE-NEGATIVE.--To understand the principle, it may
be stated that all metals are electrically positive-negative to each
other. You will remember that it has hereinbefore been stated that if,
for instance, iron and copper are put into an acid solution, a current
will be created or generated thereby. So with zinc and copper, the usual
primary battery elements. In all such cases an electrolyte is used.

Thermo-electricity dispenses with the electrolyte, and nothing is used
but the metallic elements and heat. The word thermo means heat. If, now,
we can select two strips of different metals, and place them as far
apart as possible--that is, in their positive-negative relations with
each other, and unite the end of one with one end of other by means of a
rivet, and then heat the riveted ends, a current will be generated in
the strips. If, for instance, we use an iron in conjunction with a
copper strip, the current will flow from the copper to the iron, because
copper is positive to iron, and iron negative to copper. It is from this
that the term positive-negative is taken.

The two metals most available, which are thus farthest apart in the
scale of positive-negative relation, are bismuth and antimony.

[Illustration: _Fig. 101._ THERMO-ELECTRIC COUPLE]

In Fig. 101 is shown a thermo-electric couple (A, B) riveted together,
with thin outer ends connected by means of a wire (C) to form a circuit.
A galvanometer (D) or other current-testing means is placed in this
circuit. A lamp is placed below the joined ends.

THERMO-ELECTRIC COUPLES.--Any number of these couples may be put
together and joined at each end to a common wire a