the midsection of the cores was pushed aside to form 

 a circular arch to permit the armature to be placed 

 between them. The magnetic circuit was completed 

 by vertical iron plates at the ends. One of the first 

 exact measurements on the efficiency of a dynamo 

 was made by John Hopkinson when he determined 

 the efficiency of the Siemens generator and found it 

 to range between 88 and 90 percent, depending on 

 the amount of current drawn.'"'' The Siemens firm 

 also constructed motoi's (fig. 77 d, e). 



Higgs and Brittle, two of the men associated with 

 William Siemens in the construction of the English 



Figure 71. — Gramme's Model A {top left), 

 Model C {above), and Model D generators 

 of 1879. From Minutes of Proceedings of 

 Institution of Civil Engineers, 1879, vol. 57, 

 pp. 126, 127. 



Siemens dynamo, obtained the results '°^ shown in 

 table 3 on the three models that the English firm 

 produced. 



J. N. Shoolbred made a comparison of the three 

 models of the Siemens machine and the three models 

 of the Gramme type d'atelier in 1878.""* See table 4. 

 As can be readily seen, the Gramme dynamo proved 

 to be superior in each of the three sizes compared. 

 Since all models presumably were tested under the 

 same conditions, and presumably without any bias, 

 these values should be more objective than the others 

 cited. 



Another exaluation of the two kinds of machines 



"' John Hopkinson, "On Electric Lighting," Proceedings of 

 the Institution of Mechanical Engineers, 1879, pp. 238-249. 



105 Higgs and Brittle, op. cit. (footnote 72). 



W6 J. N. Shoolbred, "On the Present State of Electric Light- 

 ing," Engineering, 1878, vol. 26, pp. 362-365. 



PAPER 30: DEVELOPMENT OF ELECTRICAL TECHNOLOGY IN THE 19TH CENTURY: III 



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