_, flush
switch; _I_, _J_, _K_, nichrome resistance wire, No. 24 (total length
of loop, 6 feet), passing around porcelain posts at left.]

The flush switch (G) should be open at the bottom for
inspection,--remove the back. The snap switch (E) should have cover
removed so that pupils can see exactly how it works.

The fuse gap (D) consists either of two parts of an old knife switch,
the knife removed, or of two brass binding posts. Across it a piece
of 4-ampere fuse wire is always kept as a protection to the more
expensive plug and cartridge fuses. Between the resistance wire (_I_,
_J_, _K_) and the wall should be either slate or sheet asbestos,
double thickness. Under the fuse gap the table should be protected by
galvanized iron so that the melted bits of fuse wire can set nothing
on fire when the fuse wire burns out.

B. CONSTRUCTION OF THE CIGAR-BOX TELEGRAPH

The "cigar-box telegraph" shown on page 381 is made as follows: An
iron machine bolt (A) is wound with about three layers of No. 24
insulated copper magnet wire, the two ends of the wire (_B_, _B_)
projecting. The threaded end of the bolt (C) is not wound. A nut (D)
is screwed on the bolt as far down as the wire wrapping. The threaded
end is then pushed up through the hole in the top of the cigar box
as that stands on its edge. Another nut (E) is then screwed on to the
bolt, holding it in position. The bolt can now be raised or lowered
and tightened firmly in position by adjusting the two nuts (_D_ and
_E_), one above and one below the wood.

A screw eye (F), large enough to form a rest for the head of another
machine bolt (G), is screwed into the back of the box about three
fourths of an inch below the head of the suspended bolt (A). Two or
three inches away, at a slightly higher level, another screw eye (H)
is screwed into the back of the cigar box. This screw eye must have
an opening large enough to permit an iron machine bolt (G) to pass
through it easily. A nut (I) is screwed down on the threaded end of a
machine bolt until about an inch of the bolt projects beyond the nut.
This projecting part of the bolt is then passed through the screw eye
(H) and another nut (J) screwed on to it to hold it in place. This nut
must not be so tight as to prevent the free play of the bolt as its
head rises and falls under the influence of the vertical bolt.
The head of the horizontal bolt rests upon the screw eye which is
immediately below the head of the suspended