This feature becomes of great importance, since it permits small and less costly conductors, when we wish to transmit energy over a great distance, as for instance from a waterfall to a distant city. But while a high voltage is desirable for transmission, it is dangerous and difficult to control, both in the machinery in which the electrical current is generated and in the apparatus in which it is utilized. By means of transformers, however, the current may at the generating station be 'stepped up' for the trans mission line and again at the receiving end `stepped down' to a lower potential for distribu tion. As an example, the water from the melting snows on the mountains of California is used in water-wheels to drive dynamos generating elec tric current at a pressure of a few thousand volts or less. This is 'stepped up' by transformers to about :30,000 volts and is transmitted through small overhead conductors nearly 200 miles to San Francisco, where it is 'stepped down' to per haps 100 volts for ordinary incandescent light ing. See TRANSMISSION OF POWER.
The device which we have outlined is modified in many ways to meet the requirements of vari ous uses, and transformers for the same purposes are built in different forms by different manufacturers. In methods of construction nearly all standard commer cial transformers may be put in one of the two classes, shell type and core type. In the shell type of trans former, part of the iron of the magnetic circuit passes around the outside and more or less envelops the pri mary and secondary wind ings. In the core type the windings almost completely cover the iron core. Very efficient apparatus is constructed in both forms, each of which has been adopted respect ively by one of the two great electrical manufac turing concerns of the United States.
If a transformer is supplied with a con stant average alternating electromotive force and is expected to deliver a constant electromotive force from its terminals, as we have assumed in our explanation, it is spoken of as a constant potential transformer. To this class the great majority of transformers in commercial use be long. If the transformer is supplied with a con stant current and is expected to deliver a con stant current, although the voltages may vary, it is known as a series or constant currant trans former. If the secondary of a transformer is in serted in series in one of the distribution lines running from a power station and used to regu late the voltage in that line it is known as a booster or induction regulator.
Of special transformers for special purposes there are many. Polyphase tr•ansformer•s eon sist of a number of pairs of windings, one for each phase of the electrical circuit, arranged upon a common magnetic circuit. Transformers deliv
ering constant current at a varying potential when supplied with a varying current at a con stant potential are used for series are lighting from constant potential supply mains. This ap parently paradoxical result is obtained by so arranging the primary and secondary coils that the magnetic leakage, that is, the lines of force passing through one coil hut not through the other, varies with the voltage required in the secondary. This can easily be obtained if the secondary coil is made movable and so arranged that it can swing away from the primary coil, which it will readily do if the current within it increases the least, since the currents of the primary and secondary coils repel each other by virtue of the fact that they flow in opposite direc tions. Another special form of transformer hay ing a limited application is the auto-transformer, in which the primary and secondary coils are united into one. To explain this type we may suppose a coil of 200 turns of wire to be wound on an iron core with a tap taken off at the one hundredth coil. If an alternating electromotive force of 100 volts is applied to 100 turns, suf ficient alternating magnetic flux will be set up to produce a counter electromotive force of 100 volts in these 100 coils; but this will also give rise to a similar electromotive force in the re maining 100 coils, so that between the extremi ties of the 200 coils we will have a potential dif ference of nearly 200 volts. This type of trans former has many special uses. It is employed, for example, to obtain the low voltages required for starting induction motors, for balancing al ternating current, multi-wire distribution cir cuits, etc.
The capacity of a transformer is limited only by the accumulation of heat in the windings and core. For this reason transformers are often immersed in oil baths, naturally or artificially cooled, or else supplied with a blast of air from a fan.
BIBLIOGRAPHY. Fleming. The Alternating CurBibliography. Fleming. The Alternating Cur- rent Transformer (3d ed., London, 1901) ; Kapp, The Electric Transmission of Energy and Its Transformation (London, 1895) ; id., Transform ers for Single and Multiphase Currents (ib., 1896) ; Steinmetz, Theory and Calculation of Al ternating Current Phenomena (3d ed., New York, 1900) ; Jackson, Alternating Currents and Alter nating Current Machinery (New Yo•k, 1900) ; and especially the Proceedings of the American and British Institutes of Electrical Engineers, and the current electrical magazines and periodicals.