Magnetic Properties of an Electric Current

Magnets and Currents:
If a current-bearing wire is really equivalent in its magnetic powers to a magnet, it must possess all of the characteristics mentioned in the preceding Section. We know that a coiled wire through which current was flowing would attract iron filings at the two ends of the helix. That a coil through which current flows possesses the characteristics a, b, c, and d of a magnet is shown as follows: -

a, b. If a helix marked at one end with a red string is arranged so that it is free to rotate and a strong current is sent through it, the helix will immediately turn and face about until it points north and south. If it is disturbed from this position, it will slowly swing back until it occupies its characteristic north and south position. The end to which the string is attached will persistently point either north or south. If the current is sent through the coil in the opposite direction, the two poles exchange positions and the helix turns until the new north pole points north.

c. If a coil conducting a current is held near a suspended magnet, one end of the helix will be found to attract the north pole of the magnet, while the opposite end will be found to repel the north pole of the magnet. In fact, the helix will
be found to behave in every way as a magnet, with a north pole at one end and a south pole at the other. If the current is sent through the helix in the opposite direction, the north and south poles exchange places.

If the number of turns in the helix is reduced until but a single loop remains, the result is the same; the single loop acts like a flat magnet, one side of the loop always facing northward and one southward, and one face attracting the north pole of the suspended magnet and one repelling it.

d. If a wire is passed through a card and a strong current is sent through the wire, iron filings will, when sprinkled upon the card, arrange themselves in definite directions. A wire carrying a current is surrounded by a magnetic field of force.

A magnetic needle held under a current-bearing wire turns on its pivot and finally comes to rest at an angle with the current. The fact that the needle is deflected by the wire shows that the magnetic power of the wire extends into the surrounding medium.

The magnetic properties of current electricity were discovered by Oersted of Denmark less than a hundred years ago; but since that time practically all important electrical machinery has been based upon one or more of the magnetic properties of electricity. The motors which drive our electric fans, our mills, and our trolley cars owe their existence entirely to the magnetic action of current electricity.

FIG. - A helix through which current flows always points north and south, if it is free to rotate.

FIG. - A wire through which current flows is surrounded by a field of magnetic force.