Transformer Indicator Diagrams. 473 



secondary of the standard and s of the triad arranged so that 

 either can be switched on to the commutator and galvano- 

 meter. Then for any position of the commutating brushes 

 the two deflexions are proportional to the mutual inductances 

 when the resistance in circuit is fixed, or if equal deflexions 

 be obtained the inductances will be proportional to the re- 

 sistances. Obviously it will be desirable to make a number 

 of independent comparisons by varying the position of the 

 commutating brushes about the place at which the maximum 

 ordinate of the current is obtained. 



The absolute value of a mutual inductance can be deter- 

 mined by the wave-tracer in terms of a resistance and a time, 

 as follows. An alternating current is sent through a Kelvin 

 balance, and at the same time a number (usually 30) of 

 eqnispaced ordinates (7) of it embracing one full wave is 

 obtained by the wave-tracer using the mutual inductance M 

 to be measured : as 



the square root of the mean squares of Q t , which is the 

 Kelvin balance reading (B say), is X?*T/4M times the square 

 root of mean squares (7 say) of the galvanometer readings, 

 hence 



4 B ' 



T is given by the chronograph attached to the wave-tracer,, 

 and the ratio of X, the reducing factor of the galvanometer, 

 to B can be obtained as follows. Send a continuous current 

 = B through the balance and through a standard resistance 

 p (usually '1 ohm). From the terminals of p lead a shunt 

 circuit through a resistance R to the galvanometer and let 

 the deflexion of the latter be d. 



Then , _ B 



\d = p^ } 



and hence ,, _ ry 



M =^ T 457r 



4. The transformer, some of whose indicator diagrams 

 will be given, was a small experimental one of the ring type 

 of about one-half kilowatt capacity. It was used as a step- 

 up one of ratio 1 to 6 transforming from about 40 volts 

 (virtual) to 240 at about 50 periods per second. 



