A Medical Coil or shocking coil, as it is properly termed, is nothing
more or less than a small induction coil, and consists of a core, a
primary winding, a secondary winding, and an interrupter. The principle
of an induction coil and that of magnetic induction have already been
explained in Chapter V. It might be well for the readers to turn back to
pages 89-91 and reread them.
The human body possesses considerable resistance, and the voltage of one
or two ordinary cells of battery is not sufficient to overcome that
resistance and pass enough current through the body to be felt, unless
under exceptional conditions.
The simplest means employable for raising the voltage of a battery high
enough to produce a shock is the medical coil.
The first step in making such a coil is to roll up a paper tube,
five-sixteenths of an inch in diameter inside, and two and one-half
inches long. The outer end of the paper is carefully glued, so that it
will not unroll. The tube is filled with pieces of iron wire two and
one-half inches long which have been straightened by rolling between two
boards. The size of the iron wire may vary from No. 20 to No. 24 B. & S.
gauge. Enough should be slipped into the tube to pack it tightly and
admit no more.
A square block, 1 x 1 x 5-16 inches, is cut out of fiber or a
close-grained hard wood and a hole three-eighths of an inch in diameter
bored through the center. One end of the tube containing the core is
smeared with glue and slipped into the block. The end of the tube is
allowed to project through about one-sixteenth of an inch. A second
block, in the form of a circle three-quarters of an inch in diameter,
one-quarter of an inch thick, and having a three-eighths of an inch hole
through the center, is glued on the opposite end.
[Illustration: Fig. 154.—Details of Various Parts of a Medical Coil.]
After the glue has dried, four small holes are drilled in the square
head in the approximate positions shown by Figure 154. Four layers of
No. 22 B. & S. gauge magnet wire (it may be either silk or cotton,
double or single covered) is wound smoothly and carefully over the core.
The terminals are led out of the holes _a_ and _b_. The primary is
covered with two or three layers of paper, and then enough secondary
wound on to bring the total diameter of the coil to about
eleven-sixteenths of an inch. The secondary wire must be much finer than
the primary. It is possible to use any size from No. 32 to No. 36 B. &
S. gauge and obtain good results. The insulation may be either single
silk or single cotton.
[Illustration: Fig. 155.—Details of Interrupter for Medical Coil.]
The secondary terminals are led out through the holes _c_ and _d_. It is
perhaps a wise plan to re-enforce these leads with a heavier piece of
wire, because otherwise they are easily broken.
The interrupter is a simple arrangement capable of being made in several
different ways. The drawing shows an arrangement which can be improved
upon by any experimenters who are familiar with a medical coil. I have
shown the simplest arrangement, so that all my readers will be able to
build it, and those who want to improve it can do so.
If a small piece of silver is soldered to the spring and to the
contact-point it will give better results. The silver is easily secured
by cutting up a ten-cent piece. One terminal of the primary is connected
to the interrupter spring and the other to a binding-post. The
contact-post is also connected to a binding-post. If a battery is
connected to the two binding-posts, the current will flow from one post
through the coil to the interrupter spring, through the spring to the
contact post, and thence back to the battery, making a complete circuit.
As soon as the current flows, however, it produces magnetism which draws
the spring away from the contact and breaks the circuit, cutting off the
magnetic pull. The spring flies back to the contact but is drawn forward
again immediately and repeats the operation continuously at a high rate
of speed.
[Illustration: Fig. 156.—Completed Medical Coil.]
The secondary terminals are led out to two binding-posts to which are
connected two electrodes or handles by means of flexible wires. The
electrode may be made of two ordinary flat strips of sheet-metal or a
piece of tubing. In the latter case, the wires may be connected by
wedging them in with a cork. If the handles are grasped while the
battery is connected to the primary posts and the interrupter is in
operation a powerful shock will be felt. The shock may be regulated from
a weak current that can hardly be felt to a very powerful one by
providing the coil with a piece of iron tubing of about seven-eighths of
an inch inner diameter and two inches long which will slip on and oh the
coil. When the tube is all the way on, the shock is very mild, and when
all the way off, the shock is very strong. Of course any intermediate
strength may be secured at stages between the two extremes.
The current from medical coils is often prescribed by physicians for
rheumatism and nervous disorders, but must be properly applied. The coil
just described is harmless. It will give a strong shock, but the only
result is to make the person receiving it drop the handles and not be
anxious to try it again.
Spark-Coils
A "spark-coil" is one of the most interesting pieces of apparatus an
experimenter can possess. The experiments that may be performed with its
aid are varied and many.
The purpose of a "spark-coil" is to generate enormously high voltages
which are able to send sparks across an air space that ordinary currents
of low voltage could not possibly pierce. The spark-coil is the same in
principle as the small induction coils used as medical or shocking
coils, but is made on a larger scale and is provided with a condenser
connected across the terminals of the interrupter.
[Illustration: Fig. 157.—Diagram showing Essential Parts of Induction
Coil.]
It consists of a central iron core surrounded by a coil of heavy wire
called the "primary," and by a second outside winding of wire known as
the "secondary." The primary is connected to a few cells of battery in
series with an interrupter. The interrupter makes and breaks the
circuit, i. e., shuts the current on and off repeatedly.
Every time that the current is "made" or broken, a high voltage is
induced in the secondary. By means of the condenser connected across the
interrupter terminals, the current at "make" is caused to take a
considerable fraction of time to grow, while at "break," the cessation
is instantaneous. The currents induced in the secondary at break are so
powerful that they leap across the space in a brilliant torrent of
sparks.
Building a Spark-Coil
Perhaps more attempts are made by experimenters to construct a
spark-coil than any other piece of apparatus, and the results are
usually poor. A spark-coil is not hard to construct, but it requires
careful work and patience. It is not a job to be finished in a day, but
time must be liberally expended in its construction. Satisfactory
results are easily obtained by any one of ordinary mechanical ability if
patience and care are used.
Parts for spark-coils are for sale by many electrical houses, and it is
possible to purchase a set of such machine-made parts for less than the
separate materials usually cost.
For the benefit of those who might wish to build a larger coil than the
one described in the following text, a table showing the dimensions of
two other sizes will be found.
[Illustration: Fig. 158.—Empty Paper Tube, and Tube filled with Core
Wire preparatory to winding on the Primary.]
*The core* is made of very soft iron wire about No. 20 or 22 B. & S.
gauge, cut to exact length. Each piece should be six inches long. Iron
wire may be purchased from electrical supply houses already cut to
various lengths for twenty cents a pound. In view of the amount of labor
required carefully to cut each piece to length and then straighten it
out so that it will form a neat bundle, it is cheaper to purchase the
wire already cut. Such wire has been annealed, i.e., softened by
bringing to a red heat and then cooling slowly. In case the wire is
purchased at a plumbing shop or a hardware store it must be annealed
before it can be used. This is accomplished by tying the wire in a
compact bundle and placing it in a wood fire where it will grow red-hot.
When this stage is reached, cover the wire with ashes and allow the fire
to die away.
[Illustration: Fig. 159.—Illustrating the Various Steps in winding on
the Primary and fastening the Ends of the Wire.]
Cut a piece of tough wrapping paper into strips six inches long and
about five inches wide. Wrap it around a stick or metal rod one-half of
an inch in diameter, so as to form a tube six inches long and having a
diameter of one-half of an inch. Glue the inside and outside edges of
the paper so that the tube cannot unroll and then slip it off the stick.
[Illustration: Fig. 160.—Complete Primary Winding and Core.]
Fill the tube with the six-inch wires until it is packed tightly and no
more can be slipped in.
*The primary* consists of two layers of No. 18 B. & S. gauge
cotton-covered wire wound over the core for a distance of five inches.
One-half pound of wire is more than enough for one primary. The wire
must be wound on very smoothly and carefully. In order to fasten the
inside end so that it will not become loose, place a short piece of tape
lengthwise of the core and wind on two or three turns over it. Then
double the end back and complete the winding. After the first layer is
finished, give it a coat of shellac and wind on the second layer. The
end of the wire is wound with a piece of tape and fastened by slipping
through a loop of tape embedded under the last few turns. The
illustrations will explain more clearly just how this is accomplished.
The second layer is then given a coat of shellac and allowed to dry.
After it is dry, wrap about fifteen layers of paper which have been
soaked in paraffin around the primary. This operation should be
performed in a warm place, over a fire or lighted lamp where the
paraffin may be kept soft, so that the paper will go on tightly.
[Illustration: Fig. 161.—The Primary covered with Insulating Layer of
Paper ready for the Secondary.]
The coil is now ready to receive the secondary winding. The core and
primary which have been described are suit-able for a secondary giving
sparks from one-half to three-fourths of an inch long.
*The secondary* winding consists of several thousand turns of very fine
wire wound on in smooth even layers with paper between each two layers.
The following table shows the size and amount of wire required. In
addition, about two pounds of paraffin and a pad of linen paper or
typewriter paper will be required. The wire may be either enamel,
cotton, or silk insulated. Single silk-covered wire is preferred.
──────────────────────────────────────────────────────────────────
SIZE OF COIL SIZE OF WIRE AMOUNT
──────────────────────────────────────────────────────────────────
1/2 inch 36 B. & S. 10 ounces
──────────────────────────────────────────────────────────────────
1 inch 34 B. & S. 1 lb.
──────────────────────────────────────────────────────────────────
1/2 inch 34 B. & S. 2 lbs.
──────────────────────────────────────────────────────────────────
The means for supporting and turning the coil in order to wind on the
secondary may be left somewhat to the ingenuity of the young
experimenter. The following suggestion, however, is one which experience
has proved to be well worth following out, and may be applied to other
things than the construction of an induction coil. It seems to be the
nature of most boys, for some reason or other, to be unwilling to spend
time and labor on anything which will aid them in their work. They are
always in such a hurry and so anxious to see something completed that
they direct all their energy to that end rather than spend part of their
time in constructing some little device which would really lighten the
other work and go a long way towards insuring its successful completion.
I have frequently given instructions for building an induction coil and
placed particular stress upon winding the secondary, only to have such
suggestions ignored in the anxious endeavor of boys to finish the coil
as soon as possible. In every such instance the coil has been a failure.
[Illustration: Fig. 162.—Simple Winding Device for winding the
Secondary.]
The illustration shows a simple form of winder, with which the operation
of winding the secondary is a very slow one, but, on the other hand, it
is possible to do very accurate, careful winding with the aid of such a
device. The parts may all be made from wood.
The chucks fit tightly over the ends of the core so that when the handle
is turned, the coil will revolve also. The spring serves to keep the
chucks snugly against the coil ends, so that they will not slip.
From one-half to five-eighths of a pound of wire will be required to
wind the coil. A large number of strips of thin paraffined or waxed
paper must be cut five inches wide. The inside terminal, or "beginning"
end of the wire is tied around the insulating tube near the left-hand
end. The spool of wire must be placed in a position where it will
revolve freely without strain on the wire. No. 36 is very fine and
easily broken, so use the utmost care to guard against this mishap.
[Illustration: Fig. 163.—Completed Secondary Winding.]
Wind on a smooth, even layer of wire, permitting each turn to touch the
other, but none to lap over. Carry the winding to within one-half inch
of the ends of the insulating tube and then wind on two layers of the
waxed paper.
The paper must be put on smoothly and evenly, so as to afford a firm
foundation for the next layer. The wire is wrapped around with the
paper, so that the next layer starts one-half inch from the edge. A
second layer is then wound on very carefully, stopping when it comes
one-half inch from the edge. Two more layers of paper are put on, and
the process repeated, alternately winding on paper and wire until the
stated quantity of the latter has been used up. The layers of wire may
occasionally be given a coating of shellac. This is a good insulator,
and will serve to hold them together and prevent the wire from becoming
loose.
In winding the coil, remember that if at any point you allow the winding
to become irregular or uneven, the irregularity will be much exaggerated
on the succeeding layers. For this reason, do not allow any to occur. If
the wire tends to go on unevenly, wrap an extra layer of thick paper
around underneath so as to offer a smooth foundation, and you will find
the difficulty remedied.
[Illustration: Fig. 164.—Interrupter Parts.]
An efficient vibrator for a coil cannot be easily made, and it is best
to buy one which is already fitted with platinum points. The interrupter
will play a very important part in the successful working of the coil,
and its arrangement and construction are important. Interrupters like
that shown in the illustration and used for automobile will be found the
best.
The condenser may be home-made. It consists of alternate sheets of
tinfoil and paraffined paper, arranged in a pile as shown in the
illustration. The following table gives the proper sizes for condensers
for three different coils.
──────────────────────────────────────────────────────────────────
TINFOIL
SIZE OF SPARK-COIL ───────────────────────────────────────────
NO. SHEETS SIZE OF SHEETS
──────────────────────────────────────────────────────────────────
1/2 inch 50 2 x 2
──────────────────────────────────────────────────────────────────
1 inch 100 7 x 5
──────────────────────────────────────────────────────────────────
1 1/2 inch 100 8 x 6
──────────────────────────────────────────────────────────────────
The paper must be about one-half inch larger all the way around, so as
to leave a good margin. The alternate sheets of tinfoil, that is, all on
one side and all on the other, are connected.
The condenser is connected directly across the terminals of the
interrupter.
[Illustration: Fig. 165.—Condenser.]
There are various methods of mounting a coil, the most common being to
place it in a box with the interrupter at one end. Perhaps, however, one
of the neatest and also the simplest methods is to mount it in the
manner shown in the illustration.
The end-pieces are cut out of wood. No specific dimensions can be given,
because the diameter of the coils will vary somewhat according to who
winds them and how tightly they are made. The coil is enclosed in a tube
made by rolling up a strip of cardboard and then giving it a coat of
shellac. The tube may be covered by a strip of black cloth, so as to
improve its appearance.
[Illustration: Fig. 166.—Completed Coil.]
The vibrator is mounted on the end. The core projects through a hole in
the wood near the end of the vibrator spring so that the latter will be
drawn in by the magnetism of the core when the current flows. The
condenser may be placed in the hollow box which forms the base of the
coil.
The secondary terminals of the coil are mounted on a small strip of wood
bridging the two coil ends.
One terminal of the primary is connected to a binding-post mounted on
the base, and the other led to the vibrator spring. The vibrator yoke is
connected to a second binding-post on the base. One terminal of the
condenser is connected to the spring, and the other to the yoke.
Four cells of dry battery should be sufficient to run the coil and cause
it to give a good one-half-inch spark if built according to the
directions here given. The vibrator or interrupter will require
adjusting and a position of the adjusting screw will soon be found where
the coil works best.
Experiments with a Spark-Coil
*Electrical Hands.* Many extraordinary and interesting experiments may
be performed with the aid of a spark-coil.
The following experiment never fails to amuse a party of friends, and is
mystifying and weird to the ordinary person, unacquainted with the
secret of its operation.
Figure 167 shows the arrangement of the apparatus. The primary of an
ordinary one-inch spark induction coil is connected in series with a
twelve-volt battery and telephone transmitter. A small switch is
included in the circuit to break the current and prevent needless waste
of the battery when the apparatus is not in immediate use. The secondary
terminals of the induction coil are led by means of an insulated wire to
the adjoining room where they terminate in a pair of scissors, or some
other small metallic object which may be clasped in the hand.
Each of two persons, wearing dry shoes or rubber-soled slippers, grasps
the terminal of one wire in one hand. The other hand is placed flat
against the ear of a third person, with a piece of dry linen paper
intervening between the hands and the head. If a fourth person, in the
room where the induction coil is located, then closes the small switch
and speaks into the telephone transmitter, the person against whose ears
the hands are being held will hear the speech very distinctly. The
ticking of a watch held against the mouthpiece of the transmitter will
be heard with startling clearness.
[Illustration: Fig. 167.—Diagram showing how to connect the Apparatus
for the "Electric Hands" Experiment.]
The principle governing the operation of the apparatus is very simple.
Almost every experimenter is familiar with the ordinary electrical
condenser, which consists of alternate sheets of paraffined paper and
tinfoil. When this is connected to a source of electricity of high
potential, but not enough so as to puncture the paper dielectric, the
alternate sheets of tinfoil will become oppositely charged and attract
each other. If the circuit is then broken the sheets will lose their
charge and also their attraction for one another. If the tinfoil sheets
and paper are not pressed tightly together, there will be a slight
movement of the tinfoil and paper which will correspond in frequency to
any fluctuations of the charging current which may take place.
The head of the third person and the hands held against his head act
like three tinfoil sheets of a condenser, separated by two sheets of
paper. The words spoken in the transmitter cause the current to
fluctuate and the induction coil raises the potential of the current
sufficiently to charge the condenser and cause a slight vibration of the
paper dielectric. The vibrations correspond in strength and speed to
those of the voice, and so the words spoken in the transmitter are
audible to the person over whose ears the paper is pressed.
Everything about the apparatus must be as dry as possible, to insure its
successful operation. The people holding the wires in their hands should
stand on a carpeted floor. Always be very careful to tighten the
adjusting screw and block the interrupter on the coil, so that by no
means can it possibly commence to operate, or the person listening,
instead of "hearing things" will become the victim of a rather painful,
practical joke.
Geissler Tubes
The most beautiful and surprising effects may be obtained by lighting
Geissler tubes with a coil. The tubes are made in intricate and varied
patterns of special glass, containing fluorescent minerals and salts,
and are filled with different rarefied gases. When the tubes are
connected to the secondary of a spark-coil by means of a wire fastened
to the little rings at the end, and the coil is set in operation, they
light up in the most wonderful way imaginable. The rarefied gases and
minerals in the glass throw out beautiful iridescent colors, lighting up
a dark room with a weird flickering light. Every tube is usually of a
different pattern and has a combination of different colors. The most
beautiful tubes are those provided with a double wall containing a
fluorescent liquid, which heightens the color effects when the tube is
lighted.
[Illustration: Fig. 168.—Geissler Tubes.]
Eight to ten tubes may be lighted at once on an ordinary coil by
connecting them in series.
Ghost Light
If you grasp the bulb of an old incandescent electric lamp in one hand
and touch the base to one side of the secondary when the coil is in
operation the bulb will emit a peculiar greenish light in the dark.
Puncturing Paper
If you place a piece of heavy paper or cardboard between two sharp wires
connected to the secondary of a spark-coil and start the coil working,
the paper will be pierced.
A Practical Joke
This action of the coil may be made the basis of an amusing joke. Offer
a friend who may smoke cigarettes some cigarette paper which has been
prepared in the following way.
[Illustration: Fig. 169.—The Bulb will emit a Peculiar Greenish Light.]
Place several sheets of the paper on a piece of sheet-metal which is
connected to one side of the secondary. By means of an insulated handle
so that you will not get a shock, move the other wire all over the
surface of the cigarette paper. The paper will be pierced with numerous
fine holes which are so fine that they can hardly be seen.
If your friend uses any of the paper in making a cigarette and tries to
light it he will waste a box of matches without being able to get one
good puff, because the little invisible holes in the paper will spoil
the draft. Perhaps he may quit smoking altogether.
An Electric Garbage-can
[Illustration: Fig. 170.—An Electrified Garbage-can.]
If there are any dogs in your neighborhood that have a habit of
extracting things from your ash-barrel or garbage-can, place the latter
on a piece of dry wood. Lead a well insulated wire from one secondary
terminal of your coil to the can. Ground the other secondary terminal.
If you see a dog with his nose in the can press your key and start the
coil working. It will not hurt the dog, but he will get the surprise of
his life. He will go for home as fast as he can travel and will not
touch that particular can again, even if it should contain some of the
choicest canine delicacies.
Photographing an Electric Discharge
The following experiment must be conducted in a dark room with the aid
of a ruby photographic lamp, as otherwise the plates used would become
lightstruck and spoiled.
[Illustration: Fig. 171.—Jacob’s Ladder.]
Placed an ordinary photographic plate on a piece of sheet-metal with the
coated side of the plate upwards. Connect one of the secondary terminals
of the spark-coil to the piece of sheet-metal.
Then sift a thin film of dry starch powder, sulphur, or talcum through a
piece of fine gauze on the plate. Lead a sharp-pointed wire from the
other secondary terminal of the coil to the center of the plate and then
push the key just long enough to make one spark.
Wipe the powder off the plate and develop it in the usual manner of
films and plates. If you cannot do developing yourself, place the plate
back in its box and send it to some friend, or to a photographer.
The result will be a negative showing a peculiar electric discharge,
somewhat like sea-moss in appearance. No two such photographs will be
alike and the greatest variety of new designs, etc., imaginable may be
produced in this manner.
Jacob’s Ladder
Take two pieces of bare copper wire about eight inches long and bend
them at right angles. Place them in the secondary terminals of a
spark-coil as in Figure 171. Bend them so that the vertical portions are
about one-half of an inch apart at the bottom and one inch apart at the
top. Start the coil working, and the sparks will run up the wires from
the bottom to the top and appear very much like the rungs in a ladder.
X-Rays
Most young experimenters are unaware what a wonderful and interesting
field is open to the possessor of a small X-ray tube.
Small X-ray tubes which will operate satisfactorily on an inch and
one-half spark-coil may be obtained from several electrical supply
houses. They usually cost about four dollars and a half. With such a
tube and a _fluoroscope_ it is possible to see the bones in the human
hand, the contents of a closed purse, etc.
The tube is made of glass and contains a very high vacuum. The long end
of the tube contains a platinum electrode called the _cathode_. The
short end contains two electrodes called _anodes_, one perpendicular to
the tube and the other diagonal.
The tube is usually clamped in a wooden holder called an X-ray tube
stand. The tube should be so adjusted that the X-rays which are
reflected from the diagonal anode will pass off in the direction shown
by the dotted lines in Figure 174.
The fluoroscope is a cone-shaped wooden box fitted with a screen
composed of a sheet of paper covered with crystals of a chemical called
platinum-barium-cyanide.
[Illustration: Fig. 172.—An X-Ray Tube.]
The opposite end of the box is fitted with a covering of felt or velvet
which shuts off the light around the eyes and nose when you look into
the fluoroscope and hold it tightly against the face.
A fluoroscope may be purchased complete, or the platinum-barium-cyanide
screen purchased separately and mounted on a box as shown in Figure 173.
The two anodes of the tube should be connected, and led to one terminal
of a spark-coil capable of giving a spark at least one and one-half
inches long. Another wire should be led from the cathode of the tube to
the other terminal of the coil.
[Illustration: Fig. 173.—Fluoroscope.]
When it is desired to inspect any object, such as the hand, it must be
held close to the screen of the fluoroscope and placed between the
latter and the tube in the path of the X-rays. The X-rays are thrown
forth from the tube at an angle of 45 degrees from the diagonal anode.
Look into the fluoroscope and it should appear to be filled with a green
light. If not, the battery terminals connected to the primary of the
coil should be reversed, so as to send the current through in the
opposite direction.
The X-rays will cause the chemicals on the screen to light up and give
forth a peculiar green light. If the hand is held against the screen,
between the screen and the tube, the X-rays will pass through the hand
and cast a shadow on the screen. They do not pass through the bones as
easily as they do through the flesh and so will cast a shadow of the
bones in the hand on the screen, and if you look closely you will be
able to see the various joints, etc. The interrupter on the coil should
be carefully adjusted so that the light does not flicker too much.
[Illustration: Fig. 174.—How to connect an X-Ray Tube to a Spark-Coil.]
If it is desired to take X-ray pictures, a fluoroscope is unnecessary.
Turn the tube around so that the X-rays point downward.
Shut the battery current off so that the tube is not in operation until
everything else is ready.
Place an ordinary photographic plate, contained in an ordinary
plate-holder, directly under the tube with the gelatin side of the plate
upwards.
Place the hand flat on the plate and lower the tube until it is only
about three inches above the hand. Then start the coil working so that
the tube lights up and permit it to run for about fifteen minutes
without removing the hand. Then turn the current off and develop the
plate in a dark room.
It is possible to obtain a very good X-ray photograph of the hand in
this manner. Photographs showing the skeleton of a mouse, nails in a
board, coins in a purse, a bullet in a piece of wood, etc., are a few of
the other objects which make interesting pictures.
[Illustration: An X-Ray Photograph of the hand taken with the Outfit
shown in Figure 174. The arrows point to injuries to the bone of the
third finger near the middle Joint Resulting in a Stiff Joint.]