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. Applying ice or cold water to the nail heads
will reverse the current.
--Contributed by A. C. A., Chicago.

** How to Discharge a Toy Cannon by Electricity [59]

A device for discharging a toy cannon by electricity can be easily
made by using three or four dry batteries, a switch and a small
induction coil

[Illustration: Electrical Attachment for Discharging Toy Cannon]

capable of giving a 1/8-in. spark. Fasten a piece of wood, A, to
the cannon, by means of machine screws or, if there are no
trunnions on the cannon, the wood may be made in the shape of a
ring and slipped on over the muzzle. The fuse hole of the cannon
is counterbored as shown and a small hole is drilled at one side
to receive a small piece of copper wire, E. The wood screw, C,
nearly touches E and is connected to one binding post of the
induction coil. The other binding post is connected with the wood
screw, D, which conducts the current into the cannon, and also
holds the pieces of wood, A and B, in position.

When the cannon is loaded, a small quantity of powder is placed in
the counterbore, and the spark between C and E ignites this and
discharges the cannon. A cannon may be fired from a distance in
this way, and as there is no danger of any spark remaining after
the current is shut off, it is safer than the ordinary cannon
w