CONTINUOUS-CURRENT DYNAMOS. 



145 



sisted on the dynamos constructed for their use being 

 rendered practically innocuous in this respect by their being 

 designed specially so as to have a negligible external field. In 

 the case of small dynamos, which require proportionately small 

 excitation, the difficulty may be met by the use of several 

 comparatively thin iron screens enclosing the dynamo and 

 separated from one another by intervening air-spaces. 

 Such a device has been employed in a dynamo specially 

 constructed for the Greenwich Observatory, 1 where delicate 

 tests showed that its working- did not affect the neighbouring; 

 instruments. When, however, we have to deal with large 

 dynamos of the ordinary two-pole type, the remedy is by 

 no means easily found, and it would seem that the only 



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Fig. 2. — Back and cross Ampere-turns of Armature. 



solution lies in a large reduction of the exciting power 

 through the use of a very small air-space between the iron 

 of the armature and the iron of the pole-pieces. 



The next point on which again it is instructive to refer 

 to the paper of Drs. J. and E. Hopkinson, is their examina- 

 tion into the reaction of the current in the armature upon 

 the magnetic field of the dynamo. Considering for simpli- 

 city a bi-polar drum armature of the ordinary type, they 

 showed that the actively inductive portion of the winding 

 which lies on the surface of the core parallel to the pole-pieces 

 has two magnetic effects which are of very different character. 

 If X (fig. 2) be the angle of lead of the brushes by which 



1 Electrician, 25th Aug., 1893. 

 10 



