used for cores

[Illustration: Mechanical Table Talk]

of induction coils, about 4 in. long and bent in the form of a
hook at the lower end, as shown at B. This wire is attached to the
block of wood, A, as shown in Fig. 2. The end of the wire is
soldered to a small brass plate which is set in the block so it
will be level or flush with the top of the block and then fastened
with two screws. The block A is fastened to the under side of the
table with two screws. A small coil, C, is made by winding No. 24
silk or cotton covered wire around a small tube, either a piece of
glass, a short straw or a quill. The coil is made tapering as
shown without using wood ends. This coil is slipped over the wire
B previous to soldering it to the small brass plate. The ends of
the coil are connected to two binding-posts which are fastened to
the block A. A small lead weight weighing 2 or 3 oz. is hung on
the hook made in the lower end of the wire B.

When all connections are made, as shown in Fig. 1, and the block
fastened to the under side of the table, the apparatus is ready
for use, and has only to be connected to an ordinary telephone
transmitter and batteries as shown. The apparatus will work to a
certain extent even if the weight is removed, though not so clear.

** Wax Wood Screws [137]

Some workmen use tallow on lag or wood screws. Try beeswax for
this purpose. It is much cleaner to use and is just as good if not
better.

** How to Make an Induction Coil [138]

A small shocking coil, suitable for medical purposes, may be
constructed of materials found in nearly every amateur mechanic's
collection of odds and ends. The core, A, Fig. 1, is a piece of
round soft iron rod about 1/4 in. in diameter and about 4 in.
long. A strip of stiff paper about 3/4 in. wide is covered with
glue and wrapped around one end of the core, as shown at B, until
the diameter is about 3/8 in. The portion of the core remaining
uncovered is then wrapped with a piece of paper about 4 in. wide.
No glue is used on this piece, as it is removed later to form the
space, C, after the paper shell, D, has been wound upon it. This
paper shell is made of stiff paper and glue the same as B and is
made about 3/64 in. thick. Two pieces of hardwood, EE, 1-3/4 in.
square and about 5/16 in. thick, are drilled in the center and
glued on the ends of the paper shell as shown.

The primary winding consists of 4 or 5 layers of No. 18 or 20
single cotton-cover