ic current interrupters, buzzers, etc., may be called
trembling armatures. They may be made entirely of sheet-tin. The part,
F, which gives it the spring, should be about 5/8 in. wide. Its length
will depend upon the particular apparatus to be made. It is made of 2
thicknesses of thin tin. See Fig. 50 for dimensions. The part N projects
beyond L. This may be used to tap against a regulating screw, or to
fasten a hammer on for an electric bell. The part, L, should have about
4 layers of tin on each side of F, and it should pinch F tightly.

[Illustration: Fig. 50.]

APPARATUS 78.

_104. Trembling Armature._ Fig. 51. When very rapid motions are desired
in a trembling armature, App. 77 will be a little heavy. A light and
quick-acting armature can be made of sheet-tin. The exact dimensions
will depend upon the use to be made of it, but you will find the
following a guide. Cut the part, B, E, out of thin tin. The covers and
bottoms of tin cans are thinner than their bodies. The narrow part, B,
should be about 1/4 in. wide and 2 in. long for a small apparatus, while
E may be 3/4 in. square. Through E is a screw, which holds it firmly to
a wooden piece, D, about 3/4 in. square. The part, E, can be made longer
than its width, so that two screws can be used; this will keep A from
jarring up or down.

[Illustration: Fig. 51.]

APPARATUS 79.

[Illustration: Fig. 52.]

_105. To File Thin Metal Strips._ Fig. 52. When sheet-metal is punched
by the methods usually employed by boys, a bulge or bur is made on the
underside around the hole. If this bur be hammered to flatten it, the
hole is distorted and made smaller. It is better to file the bur down,
at least part way. It is not convenient to file a piece of thin metal
when it is held in a vise. It is better to use either a metal or a
wooden clamp, as shown in Fig. 52; then the filing can be quickly and
easily done. Y is the yoke to be filed. It is well to place a piece of
metal, I, between the table and the end of the screw.

APPARATUS 80.

_106. Clamp._ Fig. 53. If you have no clamp to hold metal strips while
filing them, you can put a screw, S, through one hole to hold the strip
down fairly tight. Drive a nail, N, behind the strip. This will keep it
from turning while you file the free end.

[Illustration: Fig. 53.]

* * * * *

_Electro-Magnetic Armatures._ A description of this form of armature is
given in the chapter on electric