310 SCIENCE PROGRESS. 



But now a further advantage of the above method of 

 winding appears. Since the reversing E. M. F. is drawn 

 from the fringe of field at the trailing pole-tip, and this por- 

 tion of the magnetic field is strengthened by the reaction of 

 the armature current as explained in the preceding article, 

 might not this strengthening be arranged to automatically 

 furnish the stronger E. M. F. which is required to reverse 

 an increased current in the armature ? This has in fact 

 been done and Sayers-wound dynamos have been designed 

 and built in which the brushes may be left in the same 

 position from no load to full load without sparking. The 

 attendant is thus entirely relieved from any necessity to 

 shift his brushes as the load on the dynamo varies. 



The possibilities of the Sayers-wound dynamo have not 

 as yet been fully explored and delimited, but it certainly 

 holds out great promise for the future, and its invention 

 must be ranked as perhaps the most striking novelty of 

 recent years. The advantages which it possesses may be 

 briefly summed up as follows : the fixed position of the 

 brushes for all loads, the possibility of eliminating armature 

 reaction as one factor by which the output of an armature 

 is limited, and a great saving in the weight of copper and 

 iron required for a given output. This latter advantage 

 is due not only to the conversion of the back ampere-turns 

 of the armature into forward magnetising turns, but also 

 to the fact that a small air-space becomes permissible even 

 in machines delivering large currents. The Sayers method 

 of winding has been applied to a compensator giving eight 

 volts with a current of 400 amperes, 1 in which case there 

 was no field winding at all : and more recently a direct- 

 coupled dynamo of 80 kilowatts output at 420 revolutions 

 per minute 2 has been built, the total weight of which does 

 not exceed three tons. But a still more important case is 

 that of eight large dynamos which have been installed at 

 the chemical works of Mansbo in Sweden 3 for the recovery 



1 Electrical Engineer, 14th July, 1893. 



2 Electrician, 14th September, 1894. 



3 Elektrotechnische Zeitschrift, 6th September, 1894. 



