Lessons in Wireless Telegraphy

by Alfred Powell Morgan

The dynamo is perhaps the most important piece of electrical apparatus there is for it is the source of ninety-nine percent of all the electricity now in use. It is practically necessary in any case where a considerable quantity of electricity is used to have a dynamo on the spot or else bring the currents over a wire from some supply station where dynamos are kept running.

The operation of a dynamo is dependent upon current induction. It contains a system of closed conductors revolving in a magnetic field in such a way as to continuously vary the number of lines of force threading among them.

FIG. 16. Diagram showing the principle of the Dynamo.

The illustration show's the ideal simple dynamo, which consists of a loop of wire arranged to revolve between the poles of a permanent magnet in the direction of the arrow and around a horizontal line as an axis. The lines of magnetic force (represented by the fine straight lines) pass across from N to S as indicated. When in the position shown, the coil of wire encloses the largest possible number of lines of magnetic force. When it has revolved ninety degrees or a quarter of a turn as shown by the dotted lines, the lines of force will be parallel to the plane of the coil and none will pass through. During this quarter of the turn the number of lines of force has been decreasing. During the next quarter of a turn the lines will increase again, but will this time pass through from the opposite side of the loop. This decrease and increase of the number of lines of force passing through the loop generates therein a current of electricity. The same process is repeated during the next half of a revolution. However, since the lines of force flow through from opposite sides of the coil every half revolution, the current reverses twice during the same period.

In the illustration the loop is represented as forming a complete closed circuit in itself. In order to draw any current for external use some method of establishing connection to the terminals of the coil must be had. This is furnished by two circular rings called collector rings. The little strips of metal or carbon employed to form contact with the rings are called brushes.

FIG. 17. Simple Alternator.

Such a machine, so equipped will deliver alternating currents and illustrates the principle of the alternating current dynamo or alternator.

With the aid of a device called a commutator and consisting of a ring split in sections as shown in the illustration, all the successive current impulses may be turned in the same direction and the current made direct.