CONTINUOUS-CURRENT DYNAMOS. 147 



greater prominence by Mr. Swinburne : * he showed that 

 whatever the exact nature of the winding, the wires lying 

 on the surface of the core may be regarded as united across 

 the ends of the armature by imaginary connections into two 

 sets of loops, lying in planes at right angles to each other. 

 The subject was again advanced in two papers, 2 the one 

 by the last-mentioned author and the other by Mr. Esson. 

 until now the terms "back" and "cross" ampere-turns of 

 the armature have become part of the orthodox terminology 

 of the dynamo designer. 



The final result of the various theoretical points to which 

 brief allusion has been made above, may be summed up in 

 the statement that the electrical engineer is now enabled to 

 design a dynamo to produce a given voltage at a given 

 speed with such accuracy that, when built and tested, its 

 performance will not vary by more than two per cent, from 

 that predetermined by calculation : such accuracy is obtained 

 in common practice and within the limits of time that the 

 busy routine of a large engineering factory permits of being- 

 given to the design. 



The truth of the above-mentioned analysis of the effect 

 of the armature current on the magnetic field was fully 

 substantiated in another paper by Dr. Hopkinson appearing 

 in the Proceedings of the Royal Society, 1 5th Feb., 1 892. In 

 this sequel to his former paper he gave an account of some 

 experiments, in which the induction at different parts of the 

 air-gap under the bored face of the pole-piece was measured. 

 Taking two points, the one near the leading corner and the 

 other near the trailing corner, he showed that within the 

 limits of experimental error, the difference in the induction 

 at those two points corresponded exactly to the difference 

 in the magnetising forces due to the cross effect of the 

 armature current which was flowing- at the time of the 

 experiment. 3 It was thus proved that, provided the iron 



1 Journal Soc. Tel. Eng., vol. xv., No. 64, p. 542, Nov., 1886 ; also 

 "Constant-speed Motors," Electrical Engineer, 20th Jan., 1888. 



2 Journal Inst. El. Eng., vol. xix., part lxxxv. 



3 See also a paper " On the Relation of the Air-gap and the Shape of 

 the Poles to the Performance of Dynamo-electric Machinery " (Ryan), 

 American hist. El. Eng., 2nd Sept., 1892. 



