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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 shown therein.

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 coil.

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 the oscillogram.

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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