Some means of charging the condenser which produces the oscillatory
discharge is necessary. An induction coil is the most practical for the
amateur.
The induction coil consists of a primary coil of wire wound around a
central iron core and surrounded by a secondary coil consisting of many
thousand turns of carefully insulated wire. The primary coil is
connected to a source of direct current which also includes an
interrupter to "make" and "break" the current in rapid succession. Every
"make" of the circuit and consequent magnetization of the core induces a
momentary inverse current in the secondary, and every "break" and
corresponding demagnetization a momentary direct current. Normally, the
induced currents would be equal, but by means of a condenser shunted
across the interrupter the circuit when "made" requires considerable
time for the current and magnetization of the core to reach a maximum
value, while when broken the demagnetization and current drop are nearly
instantaneous. The value of the induced electromotive force in a circuit
varies as the speed at which the magnetic lines of force cut the
circuit, and so the induced e.m.f. at "break" is thus rendered high
enough to leap across a gap in the shape of sparks.
The formulas connected with induction coils depend upon conditions which
are never met in actual practice and cannot be relied upon. To construct
a coil of a given size, it is necessary to use dimensions obtained
empirically. Therefore it is well for the amateur to stick closely to
lines and hints which are given here or which appear in some _up to
date_ book on induction coil building.
[Illustration: Fig. 25. Diagram of an Induction Coil.]
For a long time the induction coil was an expensive, inefficient
instrument, until wireless telegraphy demanded of it more rigid and
efficient design and construction. It was the aim of manufacturers to
produce the longest possible spark length with a minimum amount of
secondary wire. As a result of this demand, wireless coils are now made
with a core of large diameter and give heavier and thicker sparks. The
secondary in this case is short and uses wire of large cross section in
order to reduce the resistance and minimize the heating.
No one part of an induction coil may be developed to its maximum
efficiency without seriously influencing and lowering the efficiency of
the other parts. The following suggestions regarding the construction
are given that they may prove a useful guide to the amateur coil
builder. The parts will be considered in their natural order of
construction.
*Core.*—Some experimenters not quite familiar with the principles of
magnetism reason that if an induction coil were provided with a closed
core as the transformer, the efficiency of the coil would be materially
increased. But this is not so, for the magnetization and demagnetization
of the iron cannot take place rapidly enough in a closed core when an
interrupted direct current is employed in place of an alternating
current.
The core of an induction coil is therefore always straight. For the same
reason, it is never solid but is made up of a bundle of soft iron wires
in order that rapid changes in magnetism may take place. The wires are
always of as high a permeability¹ as possible so as to create a strong
magnetic field. Swedish or Russian iron of a good quality is the best,
as its hysteresis² losses are small. The smaller the diameter of the
wire the less will be the eddy current losses and heating, but the
permeability is also rendered less and the core will not be so
effective, as the amount of iron is thereby decreased and the oxidized
surface increased. No. 22 B. S. gauge wire is the best size for the
average core.
Wires of a good quality may be purchased already cut to various lengths.
To buy them in this form will save a great deal of the labor required in
building a core. If the wires are not quite straight they may be
straightened by rolling them, one at a time, between two boards. It is
best to reanneal the wires in the following manner. Place them in an
iron pipe and plug the ends of the pipe with clay. Then lay it in a coal
fire until the whole mass attains a red heat. The fire is then allowed
to die out gradually with the pipe and wires remaining in the ashes
until cool. When cool remove them from the pipe and rub each one with
emery paper until bright. After this cleaning, the wires are dipped in
hot water and dried. They are then dipped in a good quality of varnish
and allowed to dry again.
[Illustration: Fig. 26. Induction Coil Core.]
The varnish serves to interpose resistance to the eddy currents
generated in the core and renders the losses due to this cause much
less. A strong paper tube having an internal diameter equal to the
diameter of the finished core is made by rolling the paper on a form and
cementing with shellac. When perfectly dry. the tube is removed and the
wires tightly packed in it. The following table gives the core
dimensions for practical coils of different sizes.
*Primary Winding.*—The ratio of the number of primary turns of an
induction coil to the number of secondary turns bears no relation to the
ratio of the primary and the secondary currents. It has been found in
practice that two layers of wire wound tightly on the core constitute
the best primary. The primary should always be thoroughly shellacked or
covered with insulating varnish. Since there is almost no ventilation in
the primary the wire must be large enough to avoid all heating. A table
containing the various sizes of primary wires is given below.
In large coils, the inductance of the primary causes a "kick back" and
sparks are liable to pass between the adjacent turns. For this reason,
it is always well to use double cotton covered wire and to further
thoroughly insulate it by soaking the primary and core in a pan of
melted paraffin and allowing them to harden therein. Afterwards the pan
is slightly warmed to loosen the cake of paraffin and the excess of wax
removed by scraping with a blunt instrument so as not to injure the
wires. Paraffin contracts upon hardening, and the proper method to
impregnate a porous substance is to allow it to soak and become set in
it upon cooling.
A good method of reducing the "kick back" and also the size of the
condenser shunted across the interrupter is to form the primary of a
number of turns of smaller wire in parallel, the effect being to give a
conductivity equal to a single wire of large diameter and at the same
time to make a more compact winding of the primary on the core. This
method of winding is very desirable in large coils, as it reduces the
cross section of the primary and allows the secondary to be placed
nearer the core, where the magnetic field is the strongest.
The primary winding ought to occupy nearly the whole length of the core,
since there is no gain in carrying the end of the core very far beyond
the end of the primary, for most of the magnetic lines of force bend at
the end of the primary and return without passing through the extreme
ends of the core.
*Insulating Tube.*—The successful operation of an induction coil without
breaking down when under strain depends largely upon the insulating tube
which separates the primary and secondary. Hard rubber tubes are perhaps
the best. A tube may be easily built up of several layers of 1/2-inch
sheet hard rubber by steaming it so as to soften it and then wrapping it
around a form. The tube should fit the primary tightly and be about one
inch shorter than the core. After the tube is in place it is poured full
of beeswax and rosin in order to fill all interstices and prevent sparks
due to the condenser effect of the windings from jumping from the inside
of the tube to the primary.
*Secondary.*—A coil used as a wireless telegraph transmitter must have
wire of large cross section in its secondary so as to obtain a heavy
disruptive discharge. Numbers 34 and 32 B. S. are generally used for
small coils and numbers 30 and 28 B. S. for large coils. Silk covered
wire is the usual practice, but enameled wire is coming into use. Cotton
covered wire takes up too much space and has poorer insulating
qualities.
Enameled wire is insulated by a coating of cellulose acetate, which has
a dielectric strength of about twice that of cotton and takes up much
less room than silk. There is, then, with enameled wire a great saving
in space, and a greater number of turns may be placed on the secondary
without increasing its mean distance from the core. The following table
shows the comparative diameters of silk and enamel covered wires
suitable in size for use on the secondaries of induction coils.
In winding enameled wire it must be taken into consideration that the
insulation of enameled wire is rigid and has no give. Consequently, to
allow for expansion, enameled wire must be more loosely wound than fiber
or silk covered wire. The occasional insertion of a layer of paper in
winding will give room for expansion and at the same time not add
greatly to the diameter.
[Illustration: Fig. 27. Theoretical and practical form of secondary.]
The length of the secondary is generally not much more than one-half the
length of the core. Coils giving sparks up to 2 inches in length may be
wound in two sections or in layer windings, but the layer winding is not
recommended for coils giving sparks over one inch. It is best in a coil
of this kind to insert an occasional layer of paper. The paper should be
well shellacked or paraffined and be of a good grade of linen. It should
project about one-quarter of an inch from the ends of the secondary as
shown by the sectional drawing in Fig. 28.
This insertion of paper increases the insulation and renders the
liability of sparks jumping from layer to layer much less, as is the
case when the layers are very long.
[Illustration: Fig. 28. Layer Winding for Small Coils.]
The secondaries of large coils are made up of "pies" or "pancakes" from
one-eighth to three-eighths of an inch in thickness. The "pies" are
separated from each other by a triple thickness of blotting paper which
has been thoroughly dried and then soaked in melted paraffin. In cutting
the blotting paper, much labor may be saved if a metal template of the
required size is first cut from sheet brass and then laid on the
blotting paper, which is cut by scoring around the edge of the template
with a sharp knife.
[Illustration: Fig. 29. Section Winder.]
The "pies" are wound in a bobbin or form such as is shown in Fig. 29.
The disks or flanges are made of sheet brass and mounted on an arbor so
that the form may be placed in a lathe or some other contrivance for
revolving it. The core is beveled in order to facilitate the removal of
a completed "pie" from the winder. The flanges of the winder are clamped
against the core by two nuts placed on either side. The "pie" is removed
by unscrewing one of the nuts and removing one of the flanges.
In winding silk covered wire it is first passed through a mixture of
beeswax and rosin or a bath of melted paraffin. The excess of wax is
removed by passing the wire through a slit made in a pad of paper or by
rubbing against a piece of felt. Fig. 30 shows such a contrivance.
[Illustration: Fig. 30. Impregnator for Silk Covered Wire.]
The wire passes from the reel over an ordinary spool down into the pan
of paraffin, out of the paraffin, over another spool, and rubs against a
piece of felt to remove the surplus paraffin. The spools are mounted
with a screw and a washer so that they will turn without friction.
The wire is guided, when winding, by the fingers. If it is wrapped with
a piece of felt and held between the thumb and forefinger it will run
without friction and not cut the fingers. It is necessary that the wire
should be closely watched for kinks, etc. which would cause the wire to
break. Oftentimes the wire is broken but is held together by the
insulation. Therefore each "pie" should be tested for continuity when
completed. This is best accomplished by means of a galvanometer and
battery. All imperfect "pies" should be rejected, as one of them would
cause serious trouble if embodied in the coil. In soldering the
secondary wires, acid _must not be used_ as it soon corrodes the fine
wires. Rosin is the best flux for this purpose. When building a small
coil with a "layer" winding it is absolutely necessary that the wire
should be wound on in smooth even layers. In a built-up secondary having
"pies" not greater than 1/4 inch in thickness such great care is not
necessary.
[Illustration: Fig. 31. Methods of Connecting the Secondary Sections.]
Fig. 31 shows the methods of connecting up the pies or pancakes. In _A_,
the inside of one section is connected to the outside of the next, and
so on. The maximum voltage which can exist between the adjacent sections
in this case is equal to the e.m.f. generated by one "pie" and is equal
throughout. In _B_, the coils are connected alternately inside and out.
The voltage ranges from zero at the points where they are connected, to
a value equal to twice the e.m.f. developed by one section. It would
seem that there would be a saving in insulation space of one-half in the
first case, but it is not so since the connecting wire passes between
the "pies" and therefore the insulation must be twice as thick or
exactly equal to that in the second case. The latter method (Fig. 31 B)
is the best and most convenient. When the "pies" are connected in this
manner the current must flow through alternate sections in opposite
directions. To accomplish this it is not necessary to wind every
alternate coil in an opposite direction, but merely to turn them around
and connect them with the direction of their windings reversed as shown
by the arrows and the bevels in Fig. 31. The connections between the
sections must be very carefully soldered.
After the secondary is assembled the coil should be placed in a tight
receptacle or tank containing melted paraffin. The tank is then
connected to an air pump or aspirator and the air exhausted. The
diminution of pressure causes any air bubbles in the windings to expand
and be pumped out. After standing a while, the pressure of the
atmosphere is readmitted and the place of the bubbles will be occupied
by paraffin which has been forced in under pressure.
*Mounting.*—A coil for wireless work is best mounted as shown in Fig. 32
and used with an independent interrupter. The coil may then be placed
under the operating table or on the wall, out of harm’s way, and the
interrupter on the table, where it is handy to the adjustment of the
operator.
[Illustration: Fig. 32. Coil Case.]
The case is simply a rectangular hardwood box large enough to contain
the completed coil. Two binding posts mounted on the side of the box
connect with the primary winding and two on the top of the box lead to
the secondary terminals. The box is filled with boiled oil or melted
paraffin and sealed up by screwing on the lid. If desirable, the
secondary binding posts may be mounted on the top of a short piece of
hard rubber rod as illustrated in the drawing.
¹ Magnetic permeability is the conducting power for lines of
magnetic force.
² See _hysteresis_ under Magnetic Detector.