TKANSFOKMEK. 415 TRANS-ISTHMIAN CANAL. Fig. 3, CORE TVPE TRANSFORM KR. GEN- ERAL ELECTRIC CO. PATTERN. mission line and again at the receiving end 'stepped down' to a lower potential for distribu- tion. As an example, tlio water from the melting snows on the mountains of California is used in water-wheels to drive dj'nanios generating elec- tric current at a pressure of a few thousand volts or less. This is 'stepped up' by transformers to about 60,000 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 tyjJC 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 current 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. Pohjphase transformers con- 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 ^vith a varying current at a con- stant potential are used for series arc 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 but 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 hav- 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 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 York, 1900) ; and especially the Proceedings of the American and British Institutes of Electrical Engineers, and the current electrical magazines and periodicals. TRANSFUSION OF BLOOD (Lat. trans- fusio, from transfundere, to pour from one to the other, from trans, across, through -|- fun- dere, to pour). The injection into one person of blood taken from another, either directly, from vein to vein, or after it has been defi- brinated. The operation has been regarded as legitimate in obstetric surgery since the 3'ear 1824, when Dr. Blundell published his well- known work, entitled Physiological and Patho- logical Researches. The operation had, however, been vaguely known to the medical profession for the last four centuries ; and there are obscure allusions in the Roman poets, which would seem to indicate that it was practiced as early as the Augustan age. The older operation of trans- fusion of blood has now given way to the opera- tion of infusion into the veins or subcutaneous tissues of a hot saline solution of a temperature of 100° to 120° F. This solution contains sodium chloride in the proportion in which it is found in solution in normal physiological tissues (about 0.6 per cent. ) . The amount injected varies usually from one to two pints. It is one of the most useful agents at the surgeon's command in the treatment of shock and the acute ana>mia re- sulting from sudden loss of blood. Consult: Jenning. Transfusion: Its History, etc. (Lon- don, 1883) : Zachrisson, Ex-perimentcller studier, etc. (LTpsala. 1902) ; Wallace, "Intra-venous Sa- line Injections in Shock and Hemorrhage," in Medical Revieio (Pittsburg. Pa., 1896). TRANS-ISTHMIAN CANAL. The pro- jects for a trans-isthmian canal have included
