itch, suitable for use by students of
electrical and engineering courses in performing experiments, is
shown in the diagram.

[Illustration: FIG.2 Suitable for Students' Use]

Referring to Fig. 1, A represents a pine board 4 in. by 4 in. and
a is a circular piece of wood about 1/4 in. square, with three
brass strips, b1, b2, b3, held down on it by two terminals, or
binding posts, c1, c2, and a common screw, d. Post c1 is connected
to d by means of an insulated wire, making them carry the same
kind of current (+ in the sketch).

About the center piece H moves a disk, held down by another disk F
(Fig. 2), which is fastened through the center piece to the wooden
base, A, by means of two wood screws. On the disk G are two brass
strips, e1 and e2, so arranged that, when handle K is turned to
one side, their one end just slips under the strips b1, b2, or b2,
b3, respectively, making contact with them, as shown in Fig. 2, at
L, while their other ends slide in two half-circular brass plates
f1, f2, held down on disk F by two other terminals, c3, c4, making
contact with them as shown at y, Fig. 2.

The action of the switch is shown in Fig. 1. Connect terminal c1
to the carbon of a battery, and c2 to the zinc. Then, if you turn
handle K to the right, so that the strips e1 and e2 touch b1 and
b2, respectively, terminal c3 will show +, and c1 -- electricity;
vice versa, if you turn the handle to the left so that e1 and e2
touch b2 and b3, respectively, terminal c3 will show - , and c4 +
electricity.

The switch is easy to make and of very neat appearance.

** How to Receive Wireless Telegraph Messages with a Telephone
[92]

Any telephone having carbon in the transmitter (all ordinary
telephones have carbon transmitters) can be used to receive
wireless messages by simply making a few changes in the
connections and providing a suitable antenna. Connect the
transmitter and receiver in series with three dry cells and run
one wire from the transmitter to the antenna. Connect the other
transmitter wire to a water or gas pipe in order to ground it, and
then hold the receiver to your ear. Any wireless telegraph message
within a radius of one mile will cause the transmitter to act as a
coherer, thus making the message audible in the receiver.

By using an ordinary telephone transmitter and receiver and a
1/2-in. jump spark coil, a complete wireless telegraph station may
be made, which will send or receive messages for a rad