158 Prof. Slvanus P. Thompson on 



potential. When alternating currents are employed, induction- 

 coils of appropriate construction are used as transformers. 

 But the use of such currents is attended by two disadvantages: 

 namely, that alternate currents cannot be used for electro- 

 chemical purposes, and that, pending the invention of a satis- 

 factory alternate-current motor, they cannot be used for the 

 actuating motive machinery. When continuous currents are 

 used, the appropriate transformer may be one of two types 

 — (1) the motor-generator, (2) the commuting-transformer. 



The motor-generator in its primitive form consists of a 

 motor to receive the incoming current, geared mechanically 

 to a generator which produces the outgoing current. A more 

 specialized form consists of a single machine with one field - 

 magnet and two armatures ; one to receive the primary 

 current, the other to generate the out-going or secondary 

 current. If the two sets of .armature-windings are coiled 

 around the same core, each set of windings being furnished 

 with an appropriate commutator and collecting-brushes, the 

 development has reached its extreme case, the only remaining 

 point being the proper method of excitation of the field- 

 magnet. 



The commuting-transformer is a more complex apparatus, 

 and has been much misunderstood. It does not necessarily 

 involve any greater amount of sparking than any ordinary 

 dynamo. The general principle of this species of transformer 

 may be explained by reference to a particular type. Suppose 

 that an armature double-wound with a primary coil to receive 

 the incoming current, and a secondary coil to generate the 

 outgoing current, have been provided as in the previous case, 

 each coil having its appropriate commutator. To enable this 

 piece of apparatus to transform the currents, it must either be 

 allowed to rotate in a magnetic field between the poles of a 

 field-magnet, or else the field-magnet (and collecting-brushes) 

 must be arranged to rotate, in the opposite sense, around it. 

 But there is a third possible arrangement, namely to rotate 

 around it the magnetic polarity while itself remains fixed. 

 This may be done either by rotating the brushes which bring 

 in the primary current, the magnetic circuit being completed 

 by a mass of iron external to the ring, or by rotating also the 

 polarity of a fixed field-magnet constructed specially for this 

 purpose. An external fixed ring of Cramme or Pacinotti pat- 

 tern, provided with a commutator, will answer for this purpose, 

 the current being supplied by a pair of brushes which is ro- 

 tated. In such a machine obviously all the parts are stationary 

 save the revolving brushes, which must be driven by some 

 mechanical device. Various modifications of this idea have 

 from time to time been suggested. 



