For making exact measurements of electric currents the instruments just described are not suitable, as they are not sufficiently accurate; but their working shows the principle upon which currents are measured. The actual instruments used in electrical engineering and in scientific work are unfortunately too complicated to be described here.
CHAPTER VIII
THE INDUCTION COIL
The voltaic cell and the accumulator provide us with currents of electricity of considerable volume, but at low pressure or voltage. For many purposes, however, we require a comparatively small amount of current at very high pressure, and in such cases we use an apparatus called the induction coil. Just as an electrified body and a magnet will induce electrification and magnetism respectively, so a current of electricity will induce another current; and an induction coil is simply an arrangement by which a current in one coil of wire is made to induce a current in another coil.
Suppose we have two coils of wire placed close together, one connected to a battery of voltaic cells, with some arrangement for starting and stopping the current suddenly, and the other to a galvanometer. As soon as we send the current through the first coil, the needle of the galvanometer moves, showing that there is a current flowing through the second coil; but the needle quickly comes back to its original position, showing that this current was only momentary. So long as we keep the current flowing through the first coil the galvanometer shows no further movement, but as soon as we stop the current the needle again shows by its movements that another momentary current has been produced in the second coil. This experiment shows us that a current induces another current only at the instant it is started or stopped, or, as we say, at the instant of making or breaking the circuit.
The coil through which we send the battery current is called the “primary coil,” and the one in which a current is induced is called the “secondary coil.” The two momentary currents in the secondary coil do not both flow in the same direction. The current induced on making the circuit flows in a direction opposite to that of the current in the primary coil; and the current induced on breaking the circuit flows in the same direction as that in the primary coil. If the two coils are exactly alike, the induced current will have the same voltage as the primary current; but if the secondary coil has twice as many turns of wire as the primary coil, the induced current will have twice the voltage of the primary current. In this way, by multiplying the turns of wire in the secondary coil, we can go on increasing the voltage of the induced current, and this is the principle upon which the induction coil works.
We may now describe the construction of such a coil. The primary coil is made of a few turns of thick copper wire carefully insulated, and inside it is placed a core consisting of a bundle of separate wires of soft iron. Upon this coil, but carefully insulated from it, is wound the secondary coil, consisting of a great number of turns of very fine wire. In large induction coils the secondary coil has thousands of times as many turns as the primary, and the wire forming it may be more than a hundred miles in length. The ends of the secondary coil are brought to terminals so that they can be connected up to any apparatus as desired.
Fig. 17.—Diagram showing working of Contact-Breaker for Induction Coil.
In order that the induced currents shall follow each other in quick succession, some means of rapidly making and breaking the circuit is required, and this is provided by an automatic contact breaker. It consists of a small piece of soft iron, A, [Fig. 17], fixed to a spring, B, having a platinum tip at C. The adjustable screw, D, also has a platinum tip, E. Normally the two platinum tips are just touching one another, and matters are arranged so that their contact completes the circuit. When the apparatus is connected to a suitable battery a current flows through the primary coil, and the iron core, F, becomes an electro-magnet, which draws A towards it. The platinum tips are thus no longer in contact and the circuit is broken. Immediately this occurs the iron core loses its magnetism and ceases to attract A, which is then moved back again by the spring B, so that the platinum tips touch, the circuit is once more completed, and the process begins over again. All this takes place with the utmost rapidity, and the speed at which the contact-breaker works is so great as to produce a musical note. There are many other types of contact-breakers, but in every case the purpose is the same, namely, to make and break the primary circuit as rapidly as possible.
The efficiency of the coil is greatly increased by a condenser which is inserted in the primary circuit. It consists of alternate layers of tinfoil and paraffined paper, and its action is like that of a Leyden jar. A switch is provided to turn the battery current on or off, and there is also a reversing switch or commutator, by means of which the direction of the current may be reversed. The whole arrangement is mounted on a suitable wooden base, and its general appearance is shown in [Fig. 18].