Resistance, and Self- Capacity of Magneto Windings. 73 



Referring once more to the curve of fig. 17, its utility 

 is not confined merely to the magneto. The method of 

 procedure can be adopted to gain some knowledge of the 

 self-resonance o£ any iron-cored coil. Since the resonance 

 curve (obtained by varying the inductance in circuit (2) 

 of fig. 2) is extremely fiat-topped in the neighbourhood of 

 the self-resonant frequency of the coil, the values of the 

 primary effective resistance can be obtained much more 

 accurately by a bridge method, It is highly probable that 

 the curve obtained would show subsidiary peaks indicating 

 minor resonance points at frequencies greater than the first 

 self-resonance. Some evidence of this was obtained during 

 the experiments, since the apparent inductance of the 

 primary was alternately positive and negative. It does 

 not follow, of course, that with an iron-cored coil the 

 frequencies at which these peaks occurred would bear any 

 definite relation to one another. 



The self-resonant frequency obtained by the methods 

 given in this section and in section (5) differs by 7 per cent. 

 This is due doubtless to the calculated value of h e2 being- 

 low, since it was 5 per cent, in defect at /= 2500. 



As a matter of interest, the self-resonance of the secondary 

 without an iron core was investigated using the apparatus of 

 fig. 2, and the value so obtained agreed with that calculated 

 from separate inductance and capacity measurements to 

 about 1 per cent. 



(13) Effective Inductance and Resistance under Damped 

 Oscillations. 



Consider an oscillatory circuit such as that shown in 

 fig. 20, in which the coil has an iron core. When the 



Fig. 20. — Oscillatory circuit with iron-cored inductance 

 and condenser. 



Le. Re. 



Ilk 



condenser discharges there will be oscillations, provided 



1 K, 2 



the usual well-known condition is satisfied, i. e, 1 . H > ., .>• 



