The Induction Coil
The induction coil is in general made up of two distinct windings or
coils which are usually arranged one over the other, having an annealed
iron wire core passing through their center, as shown in Fig. 1.
The diagram at Fig. 1 shows in a schematic manner the arrangement of an
induction coil designed to produce sparks or high voltages. Usually, at
least in wireless work, the primary, or heavy wire winding is placed over
the iron wire core. Suitable insulation, consisting of a few layers of
insulating cloth or paper, is placed over the iron core preparatory to
winding on this coil. After the primary has been completed, which
generally consists of two to three layers of comparatively heavy wire, it
is carefully insulated by winding over it several layers of insulating
cloth; in spark coils above one quarter inch rating it is preferable to
place a hard rubber tube over it.
The secondary winding is wound on over this tube, and it is usually
somewhat shorter in length than the primary.
Now, when the primary switch of such a coil is closed, the battery
current passes through the first winding on the core and magnetizes it.
This attracts the iron armature on the vibrator spring, as shown in Fig.
1, and when this spring breaks contact with the platinum tipped screw in
front of it, the circuit is opened. At this juncture there is induced in
the secondary winding a very powerful current. The spring-actuated
vibrator returns to its former position in the fraction of a second and
the process is repeated all over again.
Small induction coils used for medicinal purposes, such as the treatment
of rheumatism etc., are practically never fitted with a condenser across
the vibrator. All spark coils, however, are invariably equipped with such
a condenser, which reduces the spark at the vibrator contacts and also
greatly enhances the intensity of the induced secondary current.
It is generally considered, and is stated in most text-books on this
subject, that the voltage of the current induced in the secondary winding
will be proportional to the ratio existing between the number of turns of
wire in the secondary winding and the number of turns in the primary.
This ratio holds true for regular alternating current transformers, but
it does not hold exactly true for ordinary induction coils, as the
potential of the secondary induced current is, to a great extent,
proportional to the speed of the vibrator interruptions.
We may examine the phenomenon taking place at both the make and break of
the spark coil vibrator,, by referring to Figs. 2 and 3. As will be
evident from Fig. 2, the direction of the induced current in the
secondary is opposite to the direction of the primary current, during the
make period at the vibrator. This is in accordance with the law of Lenz,
which states that the direction of a current produced by electromagnetic
induction, is always such as to cause it to oppose the motion by which
such currents were produced. The half wave of secondary current induced
at make is not of very high value, and is termed the inverse current. The
phenomenon taking place at the break of the primary circuit vibrator or
interrupter is exhibited at Fig. 3. Here the secondary current passes in
the same direction as the primary current.
It is, moreover, of very high instantaneous value and possesses much
greater energy than the inverse half wave B, shown graphically in Fig. 4.
This may seem at first quite contradictory to the statement of Lenz's
law, but upon reflection it will be evident that when the primary circuit
is open the primary current magnetic flux is collapsing and in doing so
the flux lines are caused to cut the secondary turns in a direction
opposite to that at make of the circuit. Figs. 2 and 3 will make this
quite clear, as the expanding and contracting lines of force are clearly
From this discussion, as well as from the illustration given in Fig. 4,
it becomes evident that in the ordinary induction coil, in the medical
coil for instance, a pulsating direct current passing through the primary
winding is transformed into an unsymmetrical, alternating current in the
secondary winding; the half waves of which are not harmonious. In the
spark coil, however, where the secondary potential is sufficient to
create a disruptive spark, the direct current passing in the primary is
transformed into an unsymmetrical, alternating current in the secondary
only, when the spark gap is sufficiently short to allow the weaker, or
inverse half wave B, of the current to jump it. If the gap is too long
for the B half wave to leap across it, then the secondary current is
practically a undirectional one.
It is possible to test the polarity of the secondary terminals by means
of pole test paper or also a standard, liquid polarity indicator may be
utilized. If two pieces of fine iron wire are connected to the secondary
terminals of the spark coil, one of them will become very hot and the
other will remain cold ; the cold one being the positive terminal of the
As shown by the oscillogram Fig 4, which is that for a small spark coil
fitted with a vibrator shunt condenser, the duration of the primary
current at the break of the interrupter is quite short. The duration of
this portion of the primary current is kept as short as possible, and
aided in so doing, to a large extent, by the condenser shunted across the
vibrator. This condenser absorbs the extra or self-induced current of the
primary, which would otherwise unduly prolong the demagnetization of the
iron core. The general wave form of the primary current, and sensibly
also its potential, is similar to that shown at Fig. 4.
When the interrupter closes the primary circuit, the primary current
rises slowly to a maximum and at the rupture at the interrupter, the
primary current and potential fall quite rapidly to zero. The quicker the
break of the interrupter and the faster the demagnetization of the iron
core, the more pronounced the intensity or potential of the secondary
induced wave, A. This is shown graphically, and in a striking manner, by
Small spark coils may be operated in the regular way from A.C. step-down
transformers. Where 110 volts A.C. or D.C. is available it is a good idea
to operate the spark coil with an electrolytic interrupter; see Fig. 5.
Small coils, such as the l/2 or 1 inch variety, should not be hooked up
direct to 110 volt circuits, but should have a suitable choke coil in
series with the primary winding and the electrolytic interrupter.
All such installations should, no matter how small, be equipped with a
kickback preventer of approved form. It is required in all cases by the
Fire Underwriter's rules governing radio installations operating on
commercial light and power circuits.
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