58 Dr. McLachlan on Effective Inductance, Effective 



(g) Dielectric Loss. 



The problem of obtaining an accurate measurement of: 

 the dielectric loss in the magneto is rather difficult owing to 

 the peculiar conditions therein. This is parti)' due to the fact 

 that in using the coil as a condenser, the charging current h;ss 

 to traverse the copper winding. This results in an ohmic loss, 

 which is rather indefinite owing to the distribution of the 

 current. All that can be done in the present instance is to 

 obtain a rough approximation, and to show that even if the 

 ohmic effects are included, the loss is not large compared with 

 that due to the iron. 



When the iron core and the brass end-plates are removed 

 from the armature coil and the inductive reactance is very 



much greater than the capacity reactance, i.e. &)L 2 J— p , 



•the coil acts as a condenser, and tfce circulating current 

 passing through the winding can be neglected in comparison 

 with the charging current. Thus, when the coil is connected 

 as shown in fig. 4, the loss which occurs is that indicated 

 above. The current at resonance is obtained using the coil, 

 as before. The coil is removed and the resonance current 

 of equal magnitude obtained by inserting a non-inductive 

 resistance in series with the thermoammeter A. The total 

 loss in the coil is equal to the loss in this resistance. Jn 

 separating out the dielectric loss the procedure given below 

 is adopted. 



Let I be the current at resonance. 



Let R.g 2 be the resistance inserted to give the same resonance 



current as the coil. 

 Total loss in Coil = Dielectric loss + Ohmic loss due to 



charging current T2 -p 



— -t ^eq- 



Assuming charging current uniformly distributed in the 



© © © J 



winding, 



the Ohmic loss = (Charging current) 2 x res. of winding 

 = (o>C 2 E) 2 R 2 



= PB »©) • 



where C 2 is the self-capacity of the coil and is the total 



capacity in circuit. 



/C 2 \ 2 

 Equivalent series resistance = ( tt) ^2 = ?V 



(charging current) \w 



