Motion is purely a relative matter, that is, an object is said to move when it changes its position with some other object regarded as stationary; it may be moving with respect to a second object, and at the same time be at rest with respect to a third object. Thus, a dory has a speed of four miles per hour in still water; if it be run up stream against a current flowing four miles per hour it would move at that speed with respect to the water, yet remain at rest with respect to the earth.
It must be evident then that motion, as stated, being a purely relative matter, it makes no difference whether the armature of a generator move with respect to the field magnets, or the field magnets move with respect to the armature, so far as inducing an electric current is concerned.
Fig. 1,393.—Marine view, showing that motion is purely a relative matter. In order that there may be motion something must be regarded as being stationary. In the above illustration a catboat is shown at anchor in a stream which is flowing at a rate of four miles per hour in the direction of the arrow. The small dory running at a speed of four miles per hour against the current is moving at that velocity relative to the current, yet is at a standstill relative to the catboat. In this instance both catboat and dory are moving with respect to the water if the latter be regarded as stationary. Again if the earth be regarded as being stationary, the two boats are at rest and the water is moving relative to the earth.
For alternators of medium and large size there are several reasons why the armature should be stationary and the field magnets revolve, as follows:
1. By making the armature stationary, superior insulation methods may be employed, enabling the generation of current at very much higher voltage than in the revolving armature type.
2. Because the difficulty of taking current at very high pressures from collector rings is avoided.
The field current only passes through the collector rings. Since the field current is of low voltage and small in comparison with the main current, small brushes are sufficient and sparking troubles are avoided.
