Instead of taking one piece of iron, as we would for an ordinary electromagnet, we take a bundle of iron wires in making an induction-coil, as these give a stronger effect. Around this bundle of wires we wrap many turns of insulated copper wire. This is called the primary coil, and the ends of this wire are to be attached to the battery.
Fig. 9
On top of, or over, this primary coil we wrap a great many turns of very fine wire, of which, as it is so fine, a great length can be used. This is called the secondary coil, and it is in this coil that the volts, or pressure, of electricity become strongest.
Above we show you a sketch of an induction-coil. (Fig. 9.)
At the left-hand side of the cut is a "circuit-breaker," which is simply a piece of iron (armature) on a spring placed opposite the iron core. This armature is made a part of the wire leading to the primary coil. When the current from the battery is sent through the wires, the core becomes magnetized and draws this armature away from a fixed contact point, thus breaking the circuit, but the spring pulls it back, again completing the circuit, and so it keeps going back and forth very rapidly with a br-r-r-ing sound.
If you were now to take hold of the ends of the secondary coil you would get a continuous series of quick shocks which would feel like pins and needles running into you.
Perhaps most of you have taken hold of the handles of a medical battery and have had shocks therefrom. In so doing, you have simply had the current from the secondary of an induction-coil. The current may be made weaker by sliding a metallic cover over part of the iron core and so shutting off part of the magnetic effect.
SPARKING COILS
While on this subject we may add that these coils will produce sparks from the two ends of the wire of the secondary coil. These sparks vary in length according to the amount of wire in the coil. Small ones are made which give a spark a quarter of an inch in length, while others are made which will give sparks 10, 12, and 16 inches in length. In the latter, however, there are many miles of wire in the secondary coil.