** Easily Made Wireless Coherer [77]

A good wireless coherer may be made with very little expense, the
only materials necessary being a glass tube, two corks: a
magnetized needle and a quantity of iron and silver filings. Push
a piece of wire through one cork and place in the bottom of the
tube, as shown in the sketch.

Pour in the filings and insert the top cork with the needle pushed
through

[Illustration: Detail of Coherer]

from above. The point of the needle should barely touch the
filings and by slightly agitating the tube the iron filings will
separate from the silver and cling to the magnetized needle, as
shown.

In operation, the device must stand on end and should be connected
in the circuit as shown in the sketch. When the electrical waves
strike the needle, the conductivity of the filings is established
and a click is heard in the receiver.
--Contributed by Carl Formhals, Garfield, Ill.

** One-Wire Telegraph Line [78]

The accompanying wiring diagram shows a telegraph system that
requires no switches and may be operated with open-circuit
batteries on a one-wire

[Illustration: Diagram of One-Wire Line]

line with ground connections at each end. Any telegraph set in
which the key makes double contact can be connected up in this
way.
--Contributed by R. A. Brown, Fairport, N. Y.

** How to Make a Water Rheostat [78]

A water rheostat may be made by fitting a brass tube with a cork,
through which a piece of wire is passed. The brass tube may be an
old bicycle hand pump, A (see sketch), filled with water. Pushing
the wire, B, down into the water increases the surface in contact,
and thus decreases the resistance. An apparatus of this kind is
suitable for regulating the current from an induction coil, when
the coil is not provided with a regulator, and by using a piece of
pipe instead of the tube, it can be used to regulate the speed of
a motor.

When the pipe is used, a piece of brass or copper rod should be
substituted for the wire, in order to increase the surface. Adding
salt to the water will decrease the resistance, and, when used
with a motor, will give a greater speed.
--Contributed by John Koehler, Ridgewood, N. J.

[Illustration: Diagram of One-Wire Line Water Rheostat]

** Electric Door-Opener [78]

A very convenient and efficient device for unlocking any door
fitted with a spring lock is shown in the accompanying sketches. A
fairly stiff spring, A, is connected