unched.
(See App. 27.) This will make the strip bulge out on the underside
around the holes. This bur, or most of it, should be filed off. (See
App. 79 for method of filing thin pieces of metal.) The resulting yoke
may be held firmly to the magnets by the use of 2 extra nuts, as in Fig.
67. Remember that the magnets must be held firmly in the yoke.

APPARATUS 72.

_98. Yoke._ The best way of making this, of course, is to cut a piece of
bar-iron the right size. For 5/16 bolts the strip of wrought iron should
be about 3/4 in. wide and 3/16 or 1/4 in. thick. Any blacksmith can make
this and punch or drill the holes. If taps and dies (tools) are at
hand, the hole may be drilled and tapped to fit the thread on the bolt.
It is very easy to make good looking apparatus if you have, and can use,
a whole machine shop full of tools. The lengths of yokes will depend
upon the special uses to be made of them.

APPARATUS 73.

_99. Yoke._ Fig. 47. The yoke, Y, is a part of a carriage. This can be
bought at a blacksmith's. The holes are already in, but it may require
some filing before the nuts of the bolt magnets will fit down firmly.

[Illustration: Fig. 47.]

APPARATUS 74.

_100. Tin Armatures_ may be made by bending together 5 or 6 thicknesses
of tin. Different forms of tin armatures are shown under telegraph
sounders; these should have a hole punched at the center; through this
is put a screw. The length of the armature will depend upon the distance
the magnets are placed apart; they should be about 3/4 in. wide.

APPARATUS 75.

_101. Nail Armatures._ Fig. 48. A nail, N, placed through a piece of
wood, A, will serve as a very simple armature. To make it a little
heavier, if necessary, a piece of annealed iron wire, W, may be wound
around N. Care should be taken to have the two parts fairly alike in
size and weight.

[Illustration: Fig. 48.]

APPARATUS 76.

_102. Wire Armatures._ Fig. 49. Annealed iron wires make good armatures.
The short lengths of wire should be straightened (See App. 28) before
binding them into a bundle. They may be held together with thread or
paraffine, until they are in place, as, for example, in a wooden piece,
A, Fig. 49. The bundle of wires should fit snugly into the hole made
through A, and the wires should be bound together at each end with wire.

[Illustration: Fig. 49.]

APPARATUS 77.

_103. Trembling Armature._ Fig. 50. Armatures to be used upon electric
bells, automat