Motion picture projection : an elementary text book (1928)

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38 MOTION PICTURE PROJECTION ing to stop the motion. In the case of water flowing in a channel we may do this by causing the water to flow under a water wheel whose resistance to turning causes it to absorb energy from the current of water. Any material substance, by virtue of its mass, can be made to act as a vehicle for transporting energy from one place to another provided only it is set into motion. In the case of the electric current, we do not need to inquire whether electricity has mass. We are concerned, in the use of electrical apparatus, with the transformation of the energy of the current into other familiar forms of enegy — heat, light, and motion. The electric current is the vehicle by which we transmit energy from the central station to the consumer, and we are not, for practical purposes, concerned with the method of carrying the energy, any more than we need to inquire into the nature of the belt by which mechanical energy is carried from one wheel to another, or into the chemical nature of the water which is furnishing the power in a hydraulic plant. The electric current itself can not be seen, felt, smelt, heard, or tasted. Its presence can be detected only by its effects — that is, by what happens when it gives up some of its energy. Thus, an electric current may give up some of its energy, and cause a motor to turn. Electrical energy has been given up, and mechanical energy takes its place. Similarly, electric energy may disappear and heat or light may appear in its place, or a chemical effect may arise. When a person feels an electric shock, it is not the current itself he feels, but the muscular contractions and other