Page:The New International Encyclopædia 1st ed. v. 19.djvu/472

* TRANSFORMER. 4l4 TRANSrOBMER. pressed electromotive force. If, however, the sec- ondary circuit be closed so that current flows therein, this condition no longer continues. The secondary current flows in a direction contrary to the primary impressed electromotive force -and alternates simultaneously with it, and so tends to neutralize the magnetizing efl'ect of any current which may flow in phase with the primary elec- tromotive force. It may be here stated that the magnetizing effect of either coil is proportional to the size of its current multiplied by its number of turns. Furthermore, since the secondary coil is required to neutralize exactly the magnetizing eftect of the primary current in phase with the applied electromotive force, the secondary cur- rent multiplied by the secondar_y turns must equal the primary current multiplied by the pri- mary turns, that is, the currents will be in- versely proportional to the number of turns in the respective coils. In the example given above, if the terminals of the secondary coil, having a potential difference of 100 volts, are connected by an electrical cir- cuit having a resistance of one ohm, 100 amperes of current will flow, the coil itself, of course, being assumed to have no resistance. The mag- netizing effect of this current of 100 amperes flowing through 10 turns of wire will be 1000 ampere turns. Since the primary has 100 turns, the current required to produce the same num- ber of ampere turns will be 10 amperes. By sup- plying 10 amperes at lOOU volts to the primary we could take 100 amperes at 100 volts from the secondary. In other words, the output of energy (volts X amperes) equals the input. The actual transformer differs from the ideal transformer just described in the following re- spects. In the first place, the coils have electrical resistance, and some electromotive force will be consumed in causing current to flow through this resistance. The result of this is that the entire potential dift'erence called for by the ratio of the numbers of turns on the coils does not appear at the secondary terminals and that some electrical energy is converted into heat. It is obviated, as much as possible, by increasing the size and conductivity and decreasing the length of the primary and secondary conductors. In the sec- ond place, all the lines of magnetic force created by the primary coil do not pass through the .sec- ondary, and vice versa. The result of this again is that the potential difference at the terminals of the secondary coil is not equal to that called for by the ratio of transformation. This effect is re- duced to a minimum by a proper distribution of the coils with respect to each other. In the third place, energy is absorbed by the iron core in consequence of the magnetic changes constantly taking place within it. This loss of energy takes two forms, one of which is represented by elec- tric currents (called eddy or Foucault currents) induced in the iron itself. This is prevented, as far as possible, by dividing the iron into laminae at right angles to the direction of flow of these currents. The second loss of encrgj' in the core is due to the fact that the magnetism of the iron lags behind the magnetizing force, that is. when the magnetizing force is increasing the flux is not as great for a given value of the force as it is for the same value of the force when the force is decreasing. (8ee Magnetism.) This effect, called hysteresis (q.v.), is avoided, as far as pos- sible bj' choosing the proper grade of iron. The practical effects of these deviations from the ideal transformer are: the first two cause a drop in the potential at the terminals of the secondary coil; this drop increases with the load and the part of it due to the resistance of the conductors rep- resents loss of electric energy which appears as heat ; the eddy currents and hysteresis in the iron core represent transformation of electric energy into heat. The first transformers were constructed by Mi- chael Faraday in England in 1831 and at nearly the same time by Joseph Henry in the United States. Faraday discovered that when the current in a coil was varied a current was induced in a near- by coil of wire, although both were stationary and did not touch. For a long time the only application of this discovery was in the making of induction coils (q.v.), Rlihmkorft' coils, and other similar small apparatus. With the intro- duction of alternating currents into electrical engineering, however, the transformer found a wide field of application. For instance, the amount of energy wasted in heat b}' a current traversing a circuit is proportional to the square of the current multiplied by the resistance of the conductor. The energy transmitted is pro- portional to the product of the current and the electromotive force. That is, if we have a high enough electromotive force we can trans- mit a given amount of energy with a very small current, and so with relatively small conductors. 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 difl5cult to control, both in the machinery in Fig. 2. SHELL TYPE TRANSFORMER, WE8TINGH0UBE PATTERN. 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-