366 



Prof. E. Wilson. 



[May 28, 



Maximum 

 average 



B at 

 centre of 

 cylinder. 



Maximum 

 average 

 B over 

 whole 

 cylinder. 



Frequency 

 with the 

 cylinder 



10-16 cm. 



diameter. 



Ergs per 

 cycle per 



cubic 

 centimetre. 



Frequency 

 with a 

 cylinder 

 0-1 cm. 



diameter. 



Watts per 



cubic 

 centimetre 

 of a 

 cylinder 

 0-1 cm. 

 diameter. 



Fleming's 

 formula 

 (2 = 0-1 cm. 







7690 



6888 



1/80 



3820 



129 



0'049 



0-037 



15000 



17050 



26200 



516 



0-338 



0-222 



1630 



4710 



1/20 



2750 



0-142 



0-282 



15500 



20000 



55 



54500 



55 



2-810 



5-07 



Turning from round wires to plates, in which the cross-section 

 normal to the lines of force has a length great as compared with its 

 thickness, Fleming,* Steinmetz,f and Thomson! agree in giving 



1*66 — for the watts per cubic centimetre. A plate would, there- 

 fore, dissipate nearly 2 -7 times as much energy in a given time as a 

 wire having a diameter equal to the thickness of the plate. 



Rotating and Alternating Magnetic Fields. 



We have seen that the results of theory are fairly well verified by 

 experiment. It remains to extend the formulae to cases actually met 

 with in the construction of dynamo electric machinery. It is reason- 

 able to . assume that the electric currents distribute themselves in 

 paths such that constant current density is preserved. The formula 



2*78 - • ~ .. may be taken to apply with a good deal of certainty in the 



10 lb /3 



case of circular plates in which the diameter is great as compared with 

 the thickness, and in which the lines of force are uniformly distributed 

 in the plane of the plate. Comparing this with the formula 

 B 2 /* 2 / 2 



1-65 — for alternating magnetic force, we may say that the 



rotating field would dissipate about 1*7 times as much energy in a 

 given time as the alternating magnetic field. It should be remem- 

 bered, however, that even when the lines of force are confined to the 

 plane of the plate, the dissipation of energy for a given average 

 induction density may be considerably reduced if the distribution is 

 such as to give the induced electromotive force a more rectangular 

 wave-form. Some time ago§ I tried to account for the loss in a 

 smooth-core drum armature, in which the magnetic field was distributed 



* See ' The Alternate Current Transformer,' vol. 2, p. 490. 

 f See Wiener's ' Dynamo-electric Machines,' p. 119. 

 X See ' Phil. Trans.,' A, vol. 187 (1896), p. 723. 

 § See ' The Electrician ' 11th October, 1895. 



