Resistance j and Self- Capacity of Magneto Windings. 47 



or 1 Hd is the conductance. This of course includes 

 dielectric hysteresis. In conducting experiments on di- 

 electric loss the armature should be taken out of the 

 housing and the iron removed. The primary circuit should 

 be open and the condenser absent. The separation of the 

 dielectric and copper losses is dealt with later. 



(5) Bridge Method of measuring Self- Capacity. 



The self-capacity at frequencies greater than 20,000- — per 

 second can be found by using a bridge of the form shown in 

 fig. 9, instead of the substitution method already outlined. 



Pig. 9. — Diagram showing- bridge connexions for the measurement of 

 self-capacity of secondary winding. 



Iron Cored 

 Differ en t/al 

 snsformer 



The circuit of fig. 9 consists of an iron-cored differential 

 transformer having two primaries ^^ ith equal numbers of 

 turns and a secondary. The primaries are connected in 

 opposition. The coil under test is connected in parallel 

 with a ya liable condenser in one arm, and a variable 

 non-inductive resistance and variable condenser in the 

 other. When balance is obtained we have C 1 + (^ = C. pro- 

 vided .&> 2 L 2 (Ci-f-C2) is very large compared with unity, 



l/o>C2, and R 2 /a> 5 2 (a> 2 L 2 2 -f R 2 2 ) is very small: al 



so 



R - = ^ + ( ( cT B3 - " 



here R 2 is the ohmic resistance of 



the copper of the secondary winding. Thus the dielectric 

 resistance for the particular conditions which obtain in 

 the magneto can be found. Since X is known, C 2 is 

 found by subtraction. In order to obtain accurate results. 

 the value of C 2 should not be too small compared with Oj. 

 At high frequencies Ci could be zero. The detector may be 

 .a telephone* with interrupter, tikker or other device to provide 



* The telephone might be troublesome owing to the noise produced 

 when the current passes through the magneto winding, also the noise 

 from the valve generating the oscillations. 



