Resistance, and Self- Capacity of Magneto Windings. 41 



oscillations in this circuit, -the effective inductance can be 

 calculated from the expression 



1 / i 



- f ~ 1'tt VL e2 C ) ' 



where C is the self-capacity * of the secondary winding- 

 plus the capacity of the variable condenser plus the capacity 

 of the voltmeter for the reading shown thereon. It is 

 evident that this method cannot be employed for frequencies 

 exceeding that at which the coil has its first self-resonance. 

 In fact the self-resonance cannot be obtained by direct mea- 

 surement owing to the capacity of the voltmeter. 



Although it is not possible to measure the secondary 

 inductance at all frequencies by the preceding method, 

 its value can be calculated approximately from that of the 

 primary, and the results at low frequencies can be compared 

 with those found experimentally. The agreement is about 

 5 per cent, at frequencies between 1500^- and 2500-^, the 

 ■calculated values being in defect. 



Let Lj = Inductance of primary winding, out of housing, 

 with air core (measured with secondary re- 

 moved). 



L 2 = Inductance of secondary winding, out of 

 housing, with air core. 



Ai = Cross- sectional area of iron core. 



A x = Cross-sectional area of primary with air core 

 (mean) . 



A 2 = Cross- sectional area of secondary with air core 

 (mean). 



L e i -= Effective inductance of primary in housing. 



L«= 2 = Effective in iuctance of secondary in housing. 



lie = Effective permeability of magnetic circuit 

 including air gaps. 



Then L el = Additional inductance due to iron + inductance 

 with air core 



= L 1 ^( / , e -l) + L 1 



= L 1 {( / , e -l)K ] + l}, (4) 



where Kj = A^/Ap 



Similarly _ _ . t _ I . TT _ " v 



J L c2 = L 2 {(^-l)h, + l}, . . . (o) 



where K 2 = A</A 2 . 



* This can be measured, as shown below. 



