1892.] 



Dynamo-Electric Machinery. 



51 



A 2 the corrected area of field. 



I the total induction through armature. 



Z 3 the mean length of lines of magetic force in magnets. 



A 3 the area of section of magnets. 



v the ratio of induction in magnets to induction in armature. 



/ the function which the magnetising force is of the induction in 

 the case of the machine actually taken from Dr. J. Hopkinson 

 on the " Magnetisation of Iron," ' Phil. Trans.,' 1885, figs. 4 

 and 5, Plate 47. 



In estimating A 2 we take the mean of the diameter of the core and 

 of the bore of the magnets 19'8 cm., and the angle subtended by the 

 pole-face 112°, and we add a fringe all round the area of the pole-face 

 •equal in width to the distance of the core from the pole-face. This 

 is a wider fringe than was used in the earlier experiments (' Phil. 

 Trans.,' p. 337), because the form of the magnets differs slightly. 

 The area, so estimated, is 906 sq. cm. 



Z 3 is taken to be 108'8 cm. 



A 3 is 435*5 sq. cm. 



v was determined by the ballistic galvanometer to be l - 47. It is to 

 be expected that, as the core is actually greater in area than the 

 magnets, v will be more nearly constant than in the earlier experi- 

 ments. It was found to be constant within the limits of errors of 

 observation. 



(vy 



y 



and the straight line B is the curve x = 2l 2 whilst the full line D 

 is the characteristic curve of the machine 



as given by calculation. 



The marks -f- indicate the results of actual observations on machine 

 No. 1, and the marks the results on machine ~No. 2, the total induc- 

 tion I being given by the equation : — 



j Potential difference in volts X 10 8 

 208 X revolutions per second 



Experiments made upon the power taken to drive the machine 

 under different conditions show that it takes about 250 watts more 

 power to turn the armature at 660 revolutions when the magnets are 

 normally excited than when they are not excited at all. The volume 

 of the core is 9465 cub. cm., or in each complete cycle the loss per 



cub. centimeter is 9465 = ^'^^ er £ s ' 



X e 2 



