794 



Popular Science Monthly 



netic action from its contact with D 

 and the current is thus suddenly stopped. 

 This instantly demagnetizes the core and 

 induces in the secondary an E. M. F., 

 which is usually high enough to cause a 

 spark to leap the gap between t and t'. 

 As soon as the core is demagnetized, the 



Common magnetic circuit 

 of laminoted iron \ 



A.C.6enerctor 



Form of a transformer core that keeps the 

 lines of force in a continuous iron path 



spring R, supporting the hammer, re- 

 stores contact with D and the operation 

 is repeated. The condenser, shown in the 

 diagram, is not an essential part of the 

 coil; but when it is introduced, it is found 

 that the length of the spark sent across the 

 air gap is considerably increased. 



The commercial transformer is a modi- 

 fled form of the induction coil. The 

 essential difference is that the core in 

 Fig. 3, instead of being straight, is bent 

 into some other form such that the 

 magnetic lines of force have a continuous 

 iron path, instead of being obliged to 

 push out into the air, as in the case of the 

 induction coil. Furthermore, it is always 

 an alternating instead of an intermittent 

 direct current which is impressed on the 

 primary P. Sending such a current 

 through the primary is equivalent to 



Core. Secondory 



Core 



^Primorij 



fig A 



Diagrammatic illustration of two general 

 classes of transformers, a core and a shell type 



magnetizing the core first in one direction, 

 then demagnetizing it, then magnetizing 

 it in the oppo.site direction, etc. The re- 

 sult of these changes in the magnetism 

 of the coil is, of course, an induced alter- 



nating current in the secondary coil S. 



If there are few turns in the primary 

 and a large number in the secondary, the 

 transformer is called a step-up trans- 

 former, because the voltage produced at 

 the secondary terminals is greater than 

 that impressed at the terminals of the 

 primary, by the ratio of the number of 

 turns of primary and secondary coils. 

 Thus, an induction coil may be said to be 

 of the step-up type. For some uses, how- 

 ever, transformers may be of the step- 

 down type. For example, 2000 volts 

 are impressed at the terminals of the 

 primary, and a lower voltage, say 100 

 volts, is obtained at the secondary termi- 

 nals. In such a case the primary will 

 have 20 times as many turns as the sec- 

 ondary, and, we call it a 20 : 1 step-down 

 transformer. 



Assuming that the losses in the trans- 



-J w 



Fig. 5 fig, 6 



The usual arrangements of coils and core 

 for a shell type polyphase transformer 



former are so small as to be negligible, 

 the same number of magnetic lines of 

 force passes through both primary and 

 secondary coils. Since the E. M. F.'s 

 in the two coils are proportional to the 

 number of lines of force multiplied by the 

 number of turns in the coil, it follows that 

 the E. M. F.'s are directly proportional 

 to the number of turns of wire upon the 

 two coils. 



Transformers are divided into two gen- 

 eral classes, namely, core transformers 

 and shell transformers. These two types 

 are illustrated diagrammatically in Fig. 4. 



Transformers for two- or three-phase 

 currents can be made by combining two 

 or three single-phase transformers into 

 one piece of apparatus. In certain cases, 

 polyphase transformers are desirable, but 

 general practice is to use two or three 

 separate single-phase transformers for 

 transforming polyphase currents. The 

 usual arrangement of coils and core for 



