is fastened to the base.

The Adjustments are made as in App. 125. By bending N a little, O can be
made to tap E properly.

The Battery for a bell that is to be used much should be an open circuit
one, such as the Leclanche, or the ordinary dry batteries. It is cheaper
to buy a dry battery than it is to make one suitable for bells. A and B
show wires that lead to the bell from the battery. One of the wires
should be passed through a push-button.

APPARATUS 127.

_220. Electric Bell._ By arranging the buzzer of App. 125 with a bell,
you can use the same for an electric bell. The part, F, should be made
long enough to extend entirely through L, and project beyond L for about
2 in. To the end of this is fastened a large bullet, or a band of tin.
(See App. 126.)

[Illustration: Fig. 102.]

APPARATUS 128.

_221. Combination Buzzer and Telegraph Sounder._ Fig. 102. This
apparatus is good for experimental purposes, where you do not wish to go
to the trouble to make two separate pieces. For the dimensions and
explanations see App. 121 and 125. There is but a slight change in App.
125 to make this.

222. Connections. The inside ends (Sec. 123) of the magnet wires are
fastened together at S. The outside ends are joined to the two
binding-posts, Y and Z, made like App. 46. A wire, P, joins Y with the
screw in T, which is a piece of stiff tin or copper, which presses down
upon the top of I. In this way a connection may always be had between I
and T. A wire, R, joins F electrically with X; it is held under the head
of the screw, G. (See App. 125 about adjustments.)

223. Operation. When you wish to use the apparatus as a buzzer, join
your battery wires to X and Z. If the current enters Z, it will pass
through the magnet coils out to Y, through P, T, I, F, and R to X. If
you use it as a telegraph sounder, join the battery wires to Y and Z.
The current will then pass simply through the coils; it will not bother
to go into P, F, etc., as it has no place it can escape. If used simply
for experimental purposes almost any cell of sufficient strength will
do. If for telegraph, use App. 9; if for buzzer, use an open circuit
cell, as, for example, a dry cell.

CHAPTER XVI.

COMMUTATORS AND CURRENT REVERSERS.

_224. Commutators and Current Reversers_ are useful in some experiments,
as, for example, those with tangent galvanometers (App. 116, 117), in
which readings are made with the current passing around the coil