Closing either key will operate both
sounders, and, as the resistance of
[Illustration: Simple Telegraph Line]
the sounders is very high, the batteries do not run down for a
long time.
--Contributed by A. D. Stoddard, Clay Center, Kan.
** How to Make a Thermo Battery [59]
A thermo battery, for producing electricity direct from heat, can
be made of a wooden frame, A, with a number of nails, B, driven in
the vertical piece and connected in series with heavy copper
wires, C. The connections should all be soldered to give good
results, as the voltage is
[Illustration: Thermo Battery]
very low and the resistance of an unsoldered joint would stop the
current. The heat may be supplied by an alcohol lamp or other
device, and the current may then be detected by means, of a simple
galvanometer consisting of a square spool of No. 14 or No. 16
single-covered wire, E, with a pocket compass, F, placed on top.
Turn the spool in a north and south direction, or parallel with
the compass needle. Then, when the nail heads are heated and the
circuit completed, the needle will swing around it at right angles
to the coils of wire.
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