570 Dr. 8. P. Thompson and Miles Walker on the 



5. The following results are from actual experiment. The 

 core and armature were those already described in rela- 

 tion to fig. 1. The iron core was wound with 140 turns of 

 copper wire, 1*22 millim. in diameter, for the purpose of first 

 determining k w 7 hen the armature was at the distances of 9*52, 

 6*35, 3*17, and millim., respectively, from the poles. The 

 resistance of the wire when heated by carrying 12 amperes 

 was 0*3 ohm, and the frequency 93 periods per second. The 

 fourth and fifth columns are worked out from the formulas 

 given above ; for instance, in the first case 



1 = 



k = 



_ i/(31)«- (12x0-3)' 



2ttx 93x12 



= 0-00434. 



0-00434 

 (140) 2 



= 0-221 xlO" 6 . 



Air-gap 

 (millimetres). 



Virtual Volts 



at terminals 



of coil. 



Virtual 

 Amperes. 



Coefficient of 



Self-induction 



Z. 



Self-induction 

 of one turn. 



k 



952 



635 



3-17 







31 

 35-6 

 46-8 

 51 



12 

 12 

 12 

 2-76 



0-00434 

 0-005 

 0-00658 

 0-033 



0-221 X 10~ 6 

 0-225 xl0~ 6 

 0-336 xlO" 6 

 1-68 XlO" 6 



Suppose it were now required to wind the iron core so as 

 to obtain 2400 ampere-turns when the armature was distant 

 9*52 millim. from the poles, using 50 volts and the same 

 frequency as before. 



Taking the equation (5) we obtain : 



50 

 W = 0-221 x 10- 6 x 2 x 3-1416 x 93 x 2400 =163 ( a PP rox -)* 



Accordingly 163 turns of wire 2*03 millimetres in diameter 

 were wound upon the core. The current found, under the 

 above conditions, to flow through the coil was 14*9 amperes, 

 as measured by a Siemens's electrodynamometer. This shows 

 that the coil produced 2430 ampere-turns, the error being 

 well within what may be expected when using commercial 

 measuring-instruments. 



Some other results obtained with this magnet are given 

 below. 



