at you have used a cast metal base, as most
bells are now made; but if you use a wooden base, the binding posts (B,
B') and the posts (F, J) are insulated from each other, and the
construction is much simplified.

It is better, in practice, to have a small spring (P, Fig. 50) between
the armature (D) and the end of the adjusting screw (I), so as to give a
return impetus to the clapper. The object of the adjusting screw is to
push and hold the armature close up to the ends of the magnets, if it
seems necessary.

If two bells are placed on the base with the clapper mounted between
them, both bells will be struck by the swinging motion of the armature.

An easily removable cap or cover is usually placed over the coils and
armature, to keep out dust.

A very simple annunciator may be attached to the bell, as shown in the
following figures:

[Illustration: _Figs. 51-54._ ANNUNCIATOR]

ANNUNCIATORS.--Make a box of wood, with a base (A) 4" x 5" and 1/2 inch
thick. On this you can permanently mount the two side pieces (B) and two
top and bottom pieces (C), respectively, so they project outwardly
4-1/2 inches from the base. On the open front place a wood or metal
plate (D), provided with a square opening (D), as in Fig. 54, near its
lower end. This plate is held to the box by screws (E).

Within is a magnet (F), screwed into the base (A), as shown in Fig. 51;
and pivoted to the bottom of the box is a vertical armature (G), which
extends upwardly and contacts with the core of the magnet. The upper end
of the armature has a shoulder (H), which is in such position that it
serves as a rest for a V-shaped stirrup (I), which is hinged at J to the
base (C). This stirrup carries the number plate (K), and when it is
raised to its highest point it is held on the shoulder (H), unless the
electro-magnet draws the armature out of range of the stirrup. A spring
(L) bearing against the inner side of the armature keeps its upper end
normally away from the magnet core. When the magnet draws the armature
inwardly, the number plate drops and exposes the numeral through the
opening in the front of the box. In order to return the number plate to
its original position, as shown in Fig. 51, a vertical trigger (M)
passes up through the bottom, its upper end being within range of one of
the limbs of the stirrup.

This is easily made by the ingenious boy, and will be quite an
acquisition to his stock of instruments. In practice, the annunciator