Fig. 1., Fig. 2., & Fig. 3.
If two wires be attached to a cell of battery B, one to the carbon or positive pole and the other to the zinc or negative pole, and their free ends be tapped together, minute sparks at C will be observed each time the wires separate (Fig. 1). If now a coil of insulated wire S be included in the circuit, Fig. 2, upon repeating the make and break of contact, the sparks will be much increased. This arises from induction, each adjacent turn of wire acting upon its neighbor. To better understand the action of induction, we will consider the following examples: Fig. 3. A is a circuit in which is the battery cell B. C is a second circuit lying close to but well insulated from circuit A. G is a galvanometer in which a magnetized needle swings right or left each time a current is passed through a coil of wire encircling it. Now, although there is no battery cell in circuit C, yet the needle will swing each time the circuit A is closed or opened; that is, each time the wire ends are touched together or separated. This swing of course indicates that a current is passing through circuit C, but as there is no battery or other source of electrical energy included in it, it is clear that it arises from the action of the current in circuit A. In point of fact, the needle swings one way when the circuit is closed and the reverse way when it is opened; but the greater swing on opening the circuit indicates the greater strength of the induced current at the moment the current ceases to flow in circuit A. Note that these current impulses are only momentary. In the case of our single coil, Fig. 2, each turn of wire acted upon itself in a similar manner to the circuit A upon circuit C.
Fig. 4., & Fig. 5.
An iron rod or bundle of iron wires, P, inserted in the coil, Fig. 4, but carefully insulated from it, will immensely increase the inductive effects and consequently the spark. This arrangement constitutes the simple primary coil used in pull-down or pendant and automatic burners. This spark is often a source of inconvenience; it appears wherever a circuit including similar coils is made and broken. In telegraph apparatus at key and relay contacts it is noticeable; in fact, the writer has used temporarily a pair of electro-magnets from a telegraph sounder and obtained spark enough to operate a gas lighting burner.
To produce a long spark which will jump across an air gap, a more complicated form of coil is needed, one which more closely corresponds to the experiment noted in Fig. 3. The simple primary coil has here another coil of finer wire, S, wound on it but carefully insulated from it (Fig. 5). This second coil, or “secondary,” has a vast number of turns of fine wire as compared with the primary, which has only comparatively few turns of coarse wire. A primary coil of 40 feet of No. 14 B. & S. copper wire would be inserted in a secondary coil of perhaps 16,000 feet of No. 36 B. & S. This secondary coil, in fact all the apparatus constituting the induction coil, must be most highly insulated, as the electromotive force of the spark is tremendous, and it would be liable to pierce its way through and into the internal winding and so destroy the apparatus. The circuit in the primary is made and broken either by a hand key or by an automatic contact-breaker at C. With a large coil, the intensity of the spark at G is such that it will jump an air-gap of from one-eighth of an inch to over three feet.[A]
[A] See Norrie, Induction Coils and Coil-Making.
This combination of coils and contact-breaker is generally known as a Ruhmkorff or intensity coil, and is shown in elevation in Fig. 6.
