CONTINUOUS-CURRENT DYNAMOS. 141 



Drs. J. and E. Hopkinson on " Dynamo Electric Ma- 

 chinery, " * — a paper which may be said to have raised 

 dynamo-designing from rule-of-thumb empiricism to the 

 level of a scientific art. In it the authors for the first 

 time laid down clearly and concisely the true method by 

 which the voltage of a machine might be predetermined 

 from its constructive details, and so accurate was their 

 reasoning that subsequent theorising has largely consisted 

 in the explanation and expansion of their conclusions. It is 

 therefore instructive to consider modern progress by the 

 light of this remarkable paper, and we may conveniently 

 use its several headings to furnish us with starting-points 

 for our outline of more recent results. 



Its chief feature was an exhaustive analysis of the mag- 

 netic circuit of the ordinary bi-polar dynamo, and this led 

 by the way to a consideration of the important question of 

 the effect of the current in the armature on the magnetic 

 field. Previous to it a number of formulae had been in- 

 vented for determining quantitatively the magnetism of a 

 given iron circuit ; one and all, however, had been tried and 

 found wanting. The Drs. Hopkinson then took the de- 

 signer back to the fountain source, Nature itself : determine 

 by experiment the magnetic quality of the iron which is to 

 be employed, and with the data thus obtained it will be 

 possible to build up from the working drawings alone a com- 

 plete predetermination of the voltage of the dynamo under 

 any given degree of excitation. If the iron is the same as 

 that experimented on, the answer must be correct. 



The first essential then is a knowledge of our iron, 

 and this now takes the form of "curves connecting 

 B and H ' for the particular class of iron used, or, 

 in other words, curves from which for any given 

 value of the magnetic difference of potential between 

 opposite faces of a centimetre cube the number of C.G.S. 

 lines which traverse it from one face to the other can be 

 read off. Much against his will the iron manufacturer has 

 been driven by the exigencies of competition to recognise 



1 Since reprinted in a collection of papers entitled Original Papers on 

 Dynamo Machinery and Allied Subjects (Whittaker & Co.). 



