CONTINUOUS-CURRENT DYNAMOS. 



Part II. 



IN the preceding article, the question of the influence of 

 the current in the armature of a dynamo upon its mag- 

 netic field was briefly discussed, and it was shown that later 

 theorists had verified and expanded the original treatment 

 of the subject by the Drs. Hopkinson in their paper on 

 " Dynamo-Electric Machinery ". Again we have to refer to 

 the same paper as containing the germ of a recent remark- 

 able invention, an account of which is reserved for the 

 present article. 



In 1886, the Drs. Hopkinson described an experiment, 

 in which a machine was run as a generator without any 

 use of its magnet winding in order to excite its field. The 

 brushes were given a backward lead, i.e., were shifted to 

 the position, no, Fig. 3, which they would ordinarily 



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Fig. 3. — Forward and cross Ampere-turns of Armature with negative lead of brushes. 



occupy in a motor ; and when the circuit was closed, the 

 residual magnetism of the iron caused a certain E. M. F. 

 to be generated in the wires under the pole-pieces. A 

 current was thus set up, and on comparing Fig. 3 with Fig. 

 2 of the last article, it will be seen that this current flowed 

 through the wires between mn or op in the opposite 



