ILLUSTRATION OF FLEMING'S RULE.

FLEMING'S RULE: "Hold the thumb and the first and the middle fingers of the right hand as nearly as possible at right angles to each other, as in Fig. 36, so as to represent three rectangular axes in space. If the thumb points in the direction of the motion, and the forefinger points along the direction of the magnetic lines, then the middle finger will point in the direction of the induced electro-motive force."

DIRECTION OF ELECTRIC CURRENT.

the conductor in the direction of positive rotation. Conversely, if you look at the negative (south) end of a solenoid, the direction of the current is that of negative rotation, and if you look at the negative end of a conductor (i. e., imagine a section without really breaking the circuit) the direction of the lines of force about it will be that of negative rotation.

Every student of electrical engineering knows that positive rotation, mathematically speaking, is opposite to the hands of a watch, and he also knows what is meant by the positive end of a magnet and the positive terminal of a conductor. Accordingly, if he remembers the rule at all, he cannot make a mistake, like substituting "right hand for "left hand," or "clockwise " for "anti-clockwise." It is a positive rule; positive and means positive rotation and positive rotation gives positive end.-B. S. LAMPHER.

FIG. 37.

ELECTRO-MAGNETIC INDUCTION.

One of the great discoveries made by Faraday was that of induction or induced currents. While experimenting with electricity and magnetism he found that if he took a wire, joined the ends and moved it rapidly in front of a magnet, a current would be induced in the wire. This action of the magnet is called electro-magnetic induction. The current is called the induction or induced current and it is upon this principle discovered by Faraday that all dynamo electric machinery is based, as well as induction coils, alternate current transformers and other electrical appliances.

When it was discovered that an electrical spark-which is a transient electric current-would not pass through a vacuum, that it would jump three feet in the air rather than bridge the eighth of an inch where there was no conducting material; it became apparent that empty space was a perfect non-conductor of electricity; nevertheless, by this process called induction, one body may become electrified by the mere presence of another electrified body without contact with it, just as a body may be heated by another body without

contact.

When a wire, charged with a heavy current of electricity is strung parallel for a considerable distance to another wire,

ELECTRO-MAGNETIC INDUCTION.

upon which there is no electric current, or a much weaker current, the strongly-charged wire excites in the weaker wire a sympathetic current, which moves in the opposite direction, but vibrates, pulsates, and in all respects reproduces the manifestation of the current in the stronger wire. This is induction. The distance at which it will be produced varies with the intensity of the current, the atmosphere and the size of the wire.

An understanding of what is meant by induction is absolutely necessary to the explanation of the alternating system of incandescent lighting. In many instances of incandescent lighting the wires from the generating plant are carried to the outside only of the buildings to be lighted. There they are placed in a coil, in close proximity to, but not in contact with, another coil wire leading into the building. From the lastnamed coil, called the induction coil,' wires are carried to the glass bulbs, which we see on every hand, in which a.e placed carbonized loops.

Under the alternating system the incandescent lights in the buildings are produced, not by the introduction into the building of the original current, but by the sympathetic or induced current described above, set up in the induction coil from the passage of the direct current on the wire and coil outside the building.

This influence, whatever its nature, by which the primary induces a current in a secondary circuit is not affected by the nterposition of any sheet of material which is non-magnetic

ELECTRO-MAGNETIC INDUCTION.

and non-conducting. A wooden board, or a sheet of cardboard, or a sheet of india-rubber, does not [prevent the induction; but a thick sheet of copper greatly diminishes the effect, and a plate of iron prevents it altogether.

Induction can occur only when the electrified body is insolated and it is impossible to insulate against it.