Medical coils and "shocking coils" are the same. The term "shocking
coil" is usually applied to the low priced medical coils which most boys
are familiar with for giving "shocks".
[Illustration: FIG. 98.—Bobbin for Medical Coil.]
A medical coil is nothing more or less than a small induction coil for
raising the voltage of a battery high enough to produce a shock. The
human body possesses so much resistance that the voltage of one or two
cells of ordinary battery is not sufficient to pass enough current
through to be felt except under exceptional conditions, as for instance
when contact is made directly with a nerve through an open cut or a
metal filling in a tooth.
A small lathe will be of much assistance in making some of the parts and
fittings for the medical coil, although it is not absolutely essential
and the ingenious young experimenter can undoubtedly find some method of
doing without it. It is not necessary to follow the dimensions given
here exactly, and a very good coil can be built by adapting spools,
binding posts, etc., which may already be on hand, although, of course,
they should be used with some sense of proportion in mind so that
nothing is carried to an extreme.
The first part which will be required is the bobbin. This is shown in
Fig. 98 and consists of a wooden spool, two and seven-sixteenth inches
long and one inch in diameter. It has a three-eighths inch hole running
all the way through the center. The dimensions of the flanges, etc., can
probably be best understood by referring directly to the illustration.
The flanges are flattened slightly on one side so that when the bobbin
is mounted the flat portion will rest on the base and give a better
support.
[Illustration: FIG. 99.—Bobbin with Winding.]
*The Primary Winding* consists of two layers of No. 24 B. & S. Gauge
single cotton covered wire. The terminals of the winding should be led
out through two small holes in the right hand head of the bobbin. Cover
the winding with two or three layers of thin shellaced paper and then
wind on the secondary. The secondary is composed of ten layers of No. 36
B. & S. Gauge single cotton covered wire. The inside end of the
secondary should be connected to the outside end of the primary. The
outside terminal is led out through a small hole in the head at the
opposite end from the primary leads.
The wire on both the primary and secondary should be wound on in smooth
even layers. The illustration in Figure 99 shows the bobbin after the
windings are finished.
*The Core* is made out of thin sheet iron rolled up into a cylinder two
and three-sixteenths inches long and nine-thirty-seconds inches in
diameter. The core should be rolled up just as tightly as possibly,
because the more iron there is in the core, within of course reasonable
limit, the better the coil will work. Thin tin plated iron cut from a
cracker box may be used for the core. The thinner it is the better. A
core of this sort, rolled up out of sheet iron will give much better
results than a core composed of a solid bar of iron.
[Illustration: FIG. 100.—Construction of the Core.]
The end of the core is slipped into an iron "core" cap. This is shown in
Figure 101. It consists of a small cylinder, seven-sixteenths of an inch
in length, cut from a piece of iron rod, three-eighths of an inch in
diameter. A hole nine-thirty-seconds of an inch in diameter and
five-sixteenths of an inch deep should be drilled in one end. This cap
is slipped over one end of the core. It should fit very tightly so that
it will not pull out.
The core is then slipped into the bobbin with the outside end of the
core cap projecting about one-sixteenth of an inch out of the end at
which the primary terminals lead out. The cap should fit into the bobbin
very snugly so that the core is held firmly in place.
Since the hole through the bobbin is three-eighths of an inch in
diameter and the core is only nine-thirty-seconds of an inch in diameter
there should be a space all around the core, except at the end where the
cap is, so that a thin brass "regulator" tube can be slipped in and out
over the tube.
[Illustration: FIG. 101.—Vibrator Parts and Core Cap.]
*The Regulator Tube* should be two and seven-sixteenths inches long and
have an inside diameter of five-sixteenths of an inch. In case the
experimenter has any difficulty in securing a tube of this size he can
easily make one by rolling a piece of sheet brass around a suitable
sized rod and soldering the edges.
When the tube is slipped over the core and moved in and out of the
bobbin, after the coil has been finished and set in operation, it will
regulate the strength of the secondary or "shocking" current. When the
tube is completely out of the bobbin, the current will be the strongest.
If a paper scale, divided into sixteenths of an inch and bearing
numbers, is pasted around the tube it will act as an indicator of the
current strength.
The base is wood and measures four and five-eighths inches long, two and
three-quarters inches wide and is one-half an inch thick. The proper
location of the various holes can be found by referring to the
illustration in Figure 103. The corners of the base should be slightly
rounded so as to improve the appearance.
Figure 104 is a top view of the coil after it has been assembled. The
bobbin is mounted a little back of the centre of the base so that the
core cap will be opposite the armature of the vibrator.
[Illustration: FIG. 102.—Regulator Tube.]
*The Vibrator Parts* are illustrated in Figures 101 and 106.
The vibrator spring is a piece of thin steel or phosphor bronze,
one-quarter of an inch wide and one and three-sixteenths inches long. A
soft iron armature, five-sixteenths of an inch in diameter and
one-eighth of an inch thick is fastened to one end of the spring. A
five-thirty-seconds inch hole is drilled in the other end and it is then
mounted in the "vibrator posts". This consists of a small brass pillar,
one-quarter of an inch in diameter and five-eighths of an inch long. The
lower end is drilled, and tapped to receive a 4-36 screw which fastens
it to the base. A saw slot one-quarter of an inch deep is made in the
top to receive the vibrator spring.
The post which holds the adjustment screw is identical with that which
supports the vibrator spring, except that a threaded hole is drilled
through the saw slot, fifteen-thirty-seconds of an inch from the bottom,
through which the screw passes. The end of the adjustment screw, which
may be an ordinary knurled thumbscrew from a binding post, should be
fitted with a "contact" point. The best contact point is a piece of
platinum, but a small piece of silver can be made to serve the purpose
very well.
[Illustration: FIG. 103.—The Base with Location of Holes.]
[Illustration: FIG. 104.—Top View of Finished Coil.]
The vibrator spring should also be fitted with a similar contact point.
[Illustration: FIG. 105.—Side View of Completed Coil.]
The medical coil is very easy to connect. Run the inside primary
terminal to the binding post on the rear left hand corner of the base.
Connect the other terminal to the vibrator post. A wire should be led
from the adjustment screw post to the binding post mounted on the
forward left hand corner of the base.
[Illustration: FIG. 106.—Vibrator Parts.]
One or two cells of battery should be sufficient to operate the coil.
Connect the battery to the two binding posts. Adjust the thumbscrew
until the contact on the end just touches the contact point on the
vibrator spring. The current will then flow through the primary winding
and magnetize the core. The magnetism of the core will draw the armature
towards it, thus pulling the contacts apart and interrupting the
current. The process will repeat itself very rapidly at a speed
depending upon the adjustment of the thumbscrew and as long as the
battery remains connected.
[Illustration: FIGS. 107 and 108.—Two Types of Handles.]
The handles for the coil are very easily made out of a piece of brass
tubing about two and one-half inches long and five-eighths of an inch in
diameter. They can also be made by rolling a piece of sheet brass or tin
into a tube and soldering the edges together. The illustration shows two
methods of making the handles and attaching the cords. The handle in the
upper part of the illustration is made from a piece of tubing. A wooden
plug or cork is fitted into one end. The wire passes through a hole in
the centre of the plug and is turned back over the edge so that when the
plug is forced into the tube, the wire will make contact with it. The
handle in the lower part of the illustration is made from one of the
brass cylinders in which the indelible ink for rubber stamps is
supplied. A hole is bored in the bottom through which the wire passes.
The end of the wire is soldered to a piece of stiff brass wire bent into
the shape of a hairpin. The ends of the wire are spread apart so that
they will spring against the inside of the tube. The connector is then
pulled up inside of the handle.
The flexible wires or "cords" as they are sometimes called, attached to
the handles, should be about eighteen inches long. The opposite ends to
the handles should be led through two holes in the base, in front of the
bobbin. The left hand cord should be connected to the vibrator post and
the right hand cord to the outside secondary terminal.
The coil is now complete, ready for operation. Connect the battery to
the binding posts. Start the interrupter and grasp one of the handles in
each hand. Draw the regulator tube out of the bobbin gradually and the
current will gradually grow stronger and stronger until it is impossible
to hold the handles. When the regulator tube is all the way in the
bobbin it is practically impossible to feel the current. Of course any
intermediate strength between the two extremes can easily be secured.
The shock from the coil is harmless. When the regulator is all the way
out, the shock is very strong. The only result, however, is to make the
person receiving it drop the handles. The current from medical coils is
often prescribed by physicians for rheumatism and nervous disorders, but
it must be properly applied in order to be of benefit.