152 SCIENCE PROGRESS. 



when the designer succeeds in reducing one of these 

 losses, he is also rendering it easier to produce a dynamo 

 which keeps cool in continuous work, and which is there- 

 fore more likely to be durable. A striking instance of this 

 may be seen in the precautions which are now taken to 

 eliminate eddy or parasitic currents, whether in the arma- 

 ture core itself or in the winding". In large machines the 

 armature bars are now very commonly composed of several 

 separate strands, lightly insulated from each other, twisted 

 together and gently compressed into a rectangular section. 1 

 By this construction any single strand continually passes 

 from one side to the other of the bar : hence, if the field in 

 which the bar is moving varies in density within the limit 

 of its breadth, each strand is partly in the stronger field and 

 partly in the weaker. Thus no one strand produces a higher 

 E.M.F. than any other, and the tendency for a local current 

 to be set up within the bar when it enters or leaves the field 

 of a pole is entirely eliminated. 



C. C. Hawkins. 



1 Crompton's patent, No. 12,880, 1886. For a valuable method of 

 experimentally measuring the amount of the eddy currents in a wound 

 armature, see Electrician, vol. xxvi., 1891, pp. 699, 700 and 740, and vol. 

 xxvii., p. 162. 



( To be continued.) 



