Donald M. Bliss.

If the electro-magnet used in the last experiment be removed, and one of the battery wires held an inch or so above a compass needle, or the floating magnetized needle referred to in the last chapter, the wire is in such a position that it is parallel to or runs north and south with the needle, as shown in Fig. 15 ; as soon as the circuit is closed and a current flows through the wire, the needle will be instantly deflected to the east or west, according to the direction of current in the wire. 'This simple experiment, first tried in 1S20 by Oestred, forms the starting-point of the electromagnet. Farraday found that this experiment

Figure 15.

was reversible; i.e., if the needle be kept from moving and a closed loop or circuit of wire moved across or towards the magnet, a current would be generated in the wire by this movement, and would last so long as the motion continued. These two great discoveries, electro-magnetism and the induced current, have done more to benefit and advance the human race than any other investigations in natural science. Without the electro-magnet and the discovery of induction, neither the telephone, telegraph, electric light or power systems, and a thousand and one practical applications of electrical science, would be known at the present time.

The student should by all means repeat this classical experiment in various ways until he has a clear understanding of the principles involved, and as the little apparatus required may easily be constructed by the amateur worker, there should be no excuse for neglecting this important branch of electrical study.

The most important piece of apparatus required is a simple galvanometer or current detector. This is shown in Fig. 16, and consists practically of a spool of insulated wire in a certain relation with the compass. The spool consists of a block of wood in the shape of a flattened spool with the dimensions as shown. The space between the heads is wound with about three ounces No. 2S cotton-covered magnet wire, the ends of the winding being connected to binding screws on each end of the spool, as shown. A cheap pocket compass is mounted on one side of the spool just over the coil. This may be held in position by screws or pins, which should be of brass, which is nonmagnetic, and should be adjusted so that the compass may be turned at any angle desired. When in use, the block should be placed so that the wire or coil is in the same direction or parallel with the needle when at rest, i.e., north and south.

In selecting a compass, see that the needle moves freely in every direction. If the compass is held level and slowly rotated, the needle should show no tendency to follow the movement, but remain pointing north and south. In addition to the galvanometer a small quantity of small iron filings and some pieces of cardboard and stiff paper should be provided; also a small horseshoe and a straight permanent magnet, made as described in previous articles.

Referring to the experiment shown in Fig. 15, it is evident that as the magnetic needle can only be deflected by the presence near it of a magnetic field or influence, and as there is no iron or other magnetic substance near the needle to cause such a movement, the presence of an electric current in the wire must create a magnetic field outside the wire in such a manner as to tend to force the needle at right angles to the flow of the current. Hence the important rule that a current of electricity, whatever its source, always produces a magnetic field at right angles to its flow.

Figure 16.

This fact may always be shown very nicely by sending a current through a wire which has been passed through a piece of cardboard, as shown in Fig. 17, and sprinkling some fine iron filings around the wire. As soon as the circuit is closed and the card tapped. gently, the filings will arrange themselves in circles around the wire, and of course at right angles to the current. The magnetic field produced by the current may therefore be conceived as forming spirals or whorls of magnetism, which are generated at the surface of the wire and extend outward in every direction,- something after the idea shown in Fig. 18, but to a much greater distance.

As the magnetized compass needle (Fig. 15) is continually under the influence of the earth's field or polarity, it will not place itself exactly at right angles to the current flowing from it, but will take up a position depending on the relative strength of the two forces, the earth's magnetism and the current; the former tending to keep it north and south, and the other at right angles to the current, or parallel to the magnetic lines of force around the wire.

Figure 17

If, instead of surrounding the wire with iron filings as shown in Fig. 17, we reverse the experiment and surround a short rod of iron with insulating wire by winding it as shown in Fig. 19, we have an elementary form of the electro-magnet. And here a convenient way of determining the polarity of a magnet in any given case may be noted. If the wire is wound on in a right-handed spiral, and the positive ends of the battery are connected as shown, or so as to flow in a right-handed direction, the end of the magnet you are looking at will be a south pole. Briefly, as S follows 11 in the alphabet, so polarity follows right-hand rotation of current. A practical electro-magnet, of course, has many turns of wire upon it, but so long as the direction of winding is not reversed, it makes but little difference whether the wire be wound uniformly from end to end of the iron core, or whether it is tapered towards the ends, or vice versa, the result and strength of the magnet is practically the same.

Figure 18.

Figure 19.

We can now proceed with some interesting and easily performed experiments illustrating the production of induced currents.

Experiment 1. Take a wooden spool having a 1/4" or 3/8" hole through it (a thread spool will do) and wind it full with No. 28 or No. 30 single cotton-covered magnet wire, leaving the ends of the wire a foot or so long. Connect these ends to the terminals of the compass galvanometer. Then close the circuit through your large electro-magnet and battery described in the January number of Amateur Work. Now move the spool to and from one of the poles of the electro-magnet, or pass it across the ends of it, and you will note that whenever the coil moves towards the magnet the compass needle will swing in one direction, and when the coil is pulled away the needle will swing in the opposite direction ; and so long as the coil is kept in motion near the poles of the electro-magnet, a current will be generated in the little coil and affect the needle, though the coil itself has no visible connection with the battery. This experiment may also be repeated with the permanent magnet, and the same results will be noted, though in a less degree, owing to the comparatively weak magnetic field of the permanent magnet.

These effects show that magnetism, however produced, has the property of generating a current under certain conditions, as well as attracting iron or other magnetic metals. These conditions are present whenever a metal or other conductor of electricity is kept in motion within the influence of a magnetic field, and the more nearly the motion of the conduction is at right angles to this field, or lines of force, as they are now called, the greater the induced current, other conditions being equal.

If, instead of moving the coil across the poles of the magnet, it is held near one of them and the battery current through the magnet be made and broken, it will be seen that when the circuit is closed the needle will be violently deflected one way and then come to rest, and when the battery current is broken it will swing even more strongly in the opposite direction. In this case we have simply reversed the conditions by keeping the coil still and quickly sending the lines of force through the coil on closing the circuit, and as quickly withdrawing them by breaking the current producing the magnetic field.

The current generated in the little coil is called the induced or secondary current, and this is the source of power developed by the dynamo in electric light and power service. These machines are so designed that the coils of wire on the revolving part, called the armature, are rotated within a powerful magnetic field, and the currents so generated led therefrom to the lamps, motors or other devices.

The induction coil and transformer operate under the same conditions as the stationary coil and interrupted current. Here no mechanical movement of the coil takes place, but the magnetic field produced by one winding, called the primary coil, is rapidly interrupted or reversed, and a secondary or induced current is generated in the other coil, or secondary winding, with each change in the magnetic field.