42 



Dr. McLachlan on Effective Inductance, Effective 



If all the factors in (3) but /i e are known, then \x e can be 

 calculated. Substituting this value in (4) it is possible to 

 calculate L e2 . 



The mean areas of the primary and secondary windings 

 used in formula? (3) and (4) are obtained by measuring the 

 self-inductances experimentally when the iron core is 

 removed. The inductance of the primary is found after 

 the secondary has been unwound. The value of a, the 

 equivalent mean radius in Lorenz's equation L = a>i 2 Q ', 

 is then ascertained by plotting an " <x " and "L" curve 

 and reading off the value of ""a" corresponding to the 

 value of " L" found by experiment, or by trial and error. 

 The above formnbe are only intended to apply when the 

 magnetic circuit is nearly closed and the leakage is not 

 very large. Approximate overall dimensions of the armature 

 windings are given in fig. 3 a. The armature used for these 

 measurements was slightly different from that mentioned in 

 section 4. 



Fig, 3 a. 



Section AB. 



Secondary — > 

 Primary > 



IE3QQQCQQQ Q 



Primary' = 150 turns. Secondary = 10 4 turns. 



Primary res. (d.e.) = 0-55 w. Secondary res. (d.c.) = 1900 w. 

 The method of measuring the effective resistance is to 

 find the voltage at resonance, reduce the capacity so that 

 the circuit is thrown out of resonance, and again find the 

 voltage. In this case as in that of the primary winding, 

 the effective resistance includes iron, dielectric, and stray 

 losses in the metal framework. 



Let V\ = voltmeter reading at resonance. 

 (\ = condenser reading at resonance 



meter. 

 I t = current reading at resonance. 

 V 2 = voltmeter reading when circuit is out of re- 

 sonance. 

 C 2 = condenser reading out of resonance including 



voltmeter. 

 j 2 _ curr ent reading when circuit is out of resonance, 

 co = pulsatance. 

 R rf — effective resistance of secondary. 



* Bulletin Bureau of Standards, vol. viii. No. 1. 



including volt- 



