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Figure 46. — Patent drawing of Holmes' alternator of 1867. From British patent specification 



2307, August 10, 1867. 



joined so as to form one continuous closed coil.'" 

 Connections from the many subdivisions of the coil 

 led to the numerous corresponding commutator bars 

 on the axis of the armature (figs. 53-55). 



While the shuttle armature of Siemens was more 

 efficient than the Alliance disk armature, much 

 current was wasted in the open-circuit coil in com- 

 mutation; that is, in the reversal of the direction of 

 the current at each revolution which led to sparking 

 at the brushes. Gramme's closed circuit coil, with 

 its many-part commutator, mitigated the problem 

 of commutation and produced a steadier output. 

 In addition, Gramme's armature had a considerable 

 advantage over the Siemens armature in that it did 

 not become excessively hot. However, since the 

 wire on the inside of the ring armature was shielded 

 by the wire on the outside, not all the' coil was useful 



" French patent 87938 (November 22, 1869; additions, 

 April n, 1870, and February 27, 1872; the first addition 

 concerned the ring armature); British patent 1668 (June 9, 

 1870). 



in producing the output current and the resistance 

 of the armature was greater than it need be. 



The most efficient armature, and the basis of the 

 modern one, is the drum armature, which was worked 

 out in March 1872 by Friedrich von Hefner-Alteneck, 

 chief engineer at the Siemens and Halske factory 

 in Berlin and first exhibited at the Vienna Exposition 

 of 1873 (fig. 56). Von Hefner-Alteneck devised an 

 armature with a method of winding that minimized 

 the unproductive end-turns that did not cut the 

 magnetic field, but his armature still retained the 

 advantage of the Gramme ring in commutation. 

 Instead of winding the wire about a torus, Von 

 Hefner-Alteneck wound the wire about the outside 

 of a drum-shaped armature. If he had threaded 

 the turns through the interior of the cylinder, it 

 would have been topologically the same as winding 

 a torus; instead, he passed the wire directly across 

 the end faces of the cylinder to a point on the opposite 

 lateral wall. This resulted in only the end-turns not 

 cutting the lines of force. The relative amount of 

 unproductive wire was further reduced by making 



372 



BULLETIN 228: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



