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HOW TO MAKE: ELECTRICAL MACHINES. 13
ered. Ifthe galvanometer is not at hand we must take our
chance; the greatest possible care must be taken in winding
the secondary wire, as this thin wire is extremely brittle.
The insulation must now be improved by plunging the whole
coll into a deep vessel large enough to contain it, which is
tull of melted paraffin. This must be placed near the fire,
s0 as to keep the wax melted, and the coils must be left in
it to soak for an hour or two. When the paraffin has thor-
oughly permeated through it it can be taken out and held
above the vessel to drain. Ifall the wax does not run off
the ends they can be scraped afterward, taking cure not to
cut the wires. The appearance of the coil is vastly improved
by a strip of velvet cut the right width, which cau be drawn
tightly and sewn in position; or the coil may be covered
With a varnish made by dissolving red sealing-wax in spirits
of wiue by the aid of a gentle heat. The coil part of the in-
strument is now complete, and’ ready to be aflixed to the
base-board by means of two small screws passing through it
_Into the discs when placed in the proper position (see
Fig. 6.)
We now approach a very important and rather intricate
piece of workmanship. It is necessary, in order that shocks
should be obtained from the coil, that the currentin the
primary wire should be stopped and started again at the
rate of several hundred times per minute, and the more
quickly the contact between the battery wire and the prim-
ary coil is made and unmade the more powerful the shock.
_In order to accomplish this a ‘‘ contact-breaker” becomes
necessary, the method of making which is as follows: .
A piece of sheet brass is taken 1 1-2 inches long by about
3-8 ofan inch at one end, gradually tapered up till it comes
Lo a point about 1-8 of an inch broad at the other; it must
be very thin, and must act.ag a spring when fastened tightly
at one end. A small piece of soft iron is soldered to the
small end of this to be attracted by the core when working.
The next thing is to fasten a small piece of platinum feil
about 1-£ of an inch square on the opposite side of the brass
to the soft iron, and a little below it (ig. 4). This is rather
C—= _}
taal
Vie 4.—HAMMER OF CONTACT-BREAKER.
Pp, Platinum foil.
I, Soft iron fastened to opposite side.
a difficult operation, as it is such a small object to solder,