CURRENT.
The general method of using electrical gen erators is to locate the power-house either near some natural source of energy such as a water fall or near the centre of distribution of some large city, preferably near a river or lake where adjacent water maybe used for cooling the con denser of the steam-engines. Starting with 1878 with the patenting of the first incandescent lamp by Thomas Edison the development of small electrical generators became rapid, the general plan being to drive the electrical gen erator with a steam-engine. Where water power is plentiful it is used in preference to steam. In using water power it should be un derstood that while the expense for coal is elim inated still there is a larger initial investment for pipe-lines, dams, reservoirs, etc., necessary, the interest on which capital forms a large yearly expense. In many places where water power generators have been instilled, due to fluctuations in the water supply, it has been necessary to reinforce them with steam-driven generators.
In the early days of commercial electrical de velopment electric generators were only of a small size consisting of a few horse power. Generators are rated in kilowatts, a kilowatt being 1,000 watts, 746 watts equalling one horse power. The size grew rapidly until at the pres ent day we have turbine-driven generators of 50,000 horse power operating in some of our large central stations.
Thomas Edison developed the direct-current system of electrical generators and distribu tion. Due to the large loss in energy in the form of heat by the electric current passing through the wires it became necessary in the early days to build, a new power-house every few miles. At one time New York City con tained a large number of small power-houses containing generators which supplied power to the subscribers in their immediate territory. Due to these limitations electric power develop ment, especially for electric lighting, would have been extremely limited in its application were it not for Nikola Tesla's developing and patent ing the alternating current system of distribu tion. Tesla's patents covered the generator, transmission lines and power apparatus at the end of the line. He patented the complete sys tem, and while it required the expenditure of large sums of money before the manufacturers could develop practical apparatus, still the pat ents were basic and controlled the art in the United States for many years. To understand
the general principal of operation of electric generators and the structural difference between direct and alternating-current machines it is necessary to go hack to the fundamental prin ciple of electromagnetic induction discovered by Michael Faraday in 1831. He noticed that when a loop of wire of several turns was re volved between the poles of an. electromagnet a flow of electric current took place in the wires of the loop. The rotation of the loop was about an axis perpendicular to the lines of force emanating from the pole.
The term electromotive force represents the pressure which forces an electric current through a circuit. In a generator its magnitude depends upon the rate of cutting of magnetic lines of force. These lines of force are hypo thetical and indicate the magnitude and direc tion of the force of a magnetic field. To con tinuously produce an electromotive force, con tinued cutting of lines of force is essential. The magnitude of the electromotive force induced in a loop which is revolving in a magnetic field is directly proportional to the rapidity with which the loop is revolved, to the number of turns of wire in the loop and to the strength of the magnetic field. The direction of the E. M. F. induced in a loop depends upon the position of the loop relative to the poles. When one of the wires of a loop passes one pole, an E. M. F. is induced in the wire which tends to send a current in one direction, this direction changing as the wire cuts the flux at the other pole. Opposite sides of the loop pass poles of opposite polarity simultaneously, a current there fore tends to flow in one direction the loop. The direction of the E. M. F. in the loop changes as the wires pass from a north to a south pole. When the loop is connected by two slip rings, as illustrated in Fig. 1. and revolved, an alternating current may be obtained by at taching to the slip rings suitable collecting de vices. The substitution of a two-part commu tator, Fig. 2, for the slip rings will rectify the current and produce a direct current, or a cur rent which flows continuously in one direction.