The copper disc in this first dynamo did not prove satisfactory, and Faraday soon substituted for it rotating coils of wire. In 1832 a dynamo was constructed in which a length of insulated wire was wound upon two bobbins having soft iron cores, and a powerful horse-shoe magnet was fixed to a rotating spindle in such a position that its poles faced the cores of the bobbins. This machine gave a fair current, but it was found that the magnet gradually lost its magnetism on account of the vibration caused by its rotation. The next step was to make the magnet a fixture, and to rotate the bobbins of wire. This was a great improvement, and the power of machines built on this principle was much increased by having a number of rotating coils and several magnets. One such machine had 64 separate coils rotating between the poles of 40 large magnets. Finally, permanent magnets were superseded by electro-magnets, which gave a much more powerful field of force.
Fig. 19.—Diagram showing principle of Dynamo producing Alternating Current.
Having seen something of the underlying principle and of the history of the dynamo, we must turn our attention to its actual working. [Fig. 19] is a rough representation of a dynamo in its simplest form. The two poles of the magnet are shown marked north and south, and between them revolves the coil of wire A¹ A², mounted on a spindle SS. This revolving coil is called the armature. To each of the insulated rings RR is fixed one end of the coil, and BB are two brushes of copper or carbon, one pressing on each ring. From these brushes the current is led away into the main circuit, and in this case we may suppose that the current is used to light a lamp.
In speaking of the induction coil we saw that the currents induced on making and on breaking the circuit flowed in opposite directions, and similarly, Faraday found that the currents induced in a coil of wire on inserting and on withdrawing his magnet flowed in opposite directions. In the present case the magnet is stationary and the coil moves, but the effect is just the same. Now if we suppose the armature to be revolving in a clockwise direction, then A¹ is descending and entering the magnetic field in front of the north pole, consequently a current is induced in the coil, and of course in the main circuit also, in one direction. Continuing its course, A¹ passes away from this portion of the magnetic field, and thus a current is induced in the opposite direction. In this way we get a current which reverses its direction every half-revolution, and such a current is called an alternating current. If, as in our diagram, there are only two magnetic poles, the current flows backwards and forwards once every revolution, but by using a number of magnets, arranged so that the coil passes in turn the poles of each, it can be made to flow backwards and forwards several times. One complete flow backwards and forwards is called a period, and the number of periods per second is called the periodicity or frequency of the current. A dynamo with one coil or set of coils gives what is called “single-phase” current, that is, a current having one wave which keeps flowing backwards and forwards. If there are two distinct sets of coils we get a two-phase current, in which there are two separate waves, one rising as the other falls. Similarly, by using more sets of coils, we may obtain three-phase or polyphase currents.
Fig. 20.—Diagram showing principle of Dynamo producing Continuous Current.
Alternating current is unsuitable for certain purposes, such as electroplating; and by making a small alteration in our dynamo we get a continuous or direct current, which does not reverse its direction. [Fig. 20] shows the new arrangement. Instead of the two rings in [Fig. 19], we have now a single ring divided into two parts, each half being connected to one end of the revolving coil. Each brush, therefore, remains on one portion of the ring for half a revolution, and then passes over on to the other portion. During one half-revolution we will suppose the current to be flowing from brush B¹ in the direction of the lamp. Then during the next half-revolution the current flows in the opposite direction; but brush B¹ has passed on to the other half of the ring, and so the current is still leaving by it. In this way the current must always flow in the same direction in the main circuit, leaving by brush B¹ and returning by brush B². This arrangement for making the alternating current into a continuous current is called a commutator.
PLATE IV.
