TECHNOLOGY. 



By WILLIAM H. WAHL, Ph.D., 



Philadelphia, Pa. 



COMPARATIVE MERITS OF DYNAMO-ELECTRIC MACHINES. 



A most valuable and practical contribution on the subject 

 of Dynamo-electric Machines was made during the last year 

 by a committee of the Franklin Institute. The report of the 

 sub -committee on Electrical Measurements, by Professors 

 Houston and Thomson, is of such special value that its con- 

 clusions may be properly condensed in this place. The ma- 

 chines that were submitted to the comparative tests were a 

 Gramme, a large Brush, a small Brush, and a Wallace -Far- 

 mer machine ; and regret is expressed by the committee that 

 it was not in their power to include a machine of the Sie- 

 mens type (which lately received a very favorable endorse- 

 ment from Professor Tyndall) in the list of those investi- 

 gated. The conclusions reached by the gentlemen above 

 named are, briefly condensed, as follows : The Gramme 

 machine, considered as a means for converting motive-power 

 into electrical current, is pronounced to be the most econom- 

 ical, giving a useful result in the arc equal to 38 per cent, 

 (of power utilized), or of 41 per cent, after deducting friction 

 and the resistance of the air. In this machine the loss by 

 friction and local action is the least, the speed being com- 

 paratively low. 



The laro;e Brush machine o-ave in the arc a useful effect 

 equal to 31 per cent, of the power used, or, after deducting 

 friction, 37^ per cent. It is but little inferior, in this respect, 

 to the Gramme, but has the disadvantage of higher speed, 

 and consequently greater percentage of loss of power by 

 friction. This loss, the committee note, is nearly compen- 

 sated by the advantage possessed by this machine over the 

 others of working with a high external compared with the 

 internal resistance, thus assuring comparative absence of 

 heating in the machine. The small Brush is ranked by the 



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