482 Mr. Rimington on the Measurement of the Power 



drawn from them is that although the watts per candle for 

 green light are not quite the same for direct and for alter- 

 nating currents, and although the volts per candle for red 

 light are also not exactly the same for direct and alternating, 

 still the difference between the results for the same colour is 

 so small that it may be put down to experimental errors ; and 

 this, combined with the fact that the mean of all the 75 experi- 

 ments gives practically the same number of watts per candle 

 for both direct and alternating currents, leads to the practical 

 certainty that the efficiency of an incandescent lamp is the same 

 for both direct and alternating currents. 



LIX. On the Measurement of the Power supplied to the Pri- 

 mary Coil of a Transformer. By E. C. Bimington*. 



IN the discussion on Mr. Kapp's paper on Transformers, 

 at the Society of Telegraph Engineers, Professor Ayrton 

 gave a formula for calculating the true power supplied to the 

 primary from the reading of a Siemens wattmeter. The 

 thick wire coil of the wattmeter is in series with the primary 



/ VWVYVYV-^ 



coil, and the fine wire coil connected as a shunt on the two 

 as in the diagram, where P is the primary coil, A the thick 

 and B the fine wire coil of the wattmeter. Let r x be the 

 resistance of the primary P including A, and L, its coefficient 

 of self-induction also including A ; let r 2 and L 2 be the resist- 

 ance and coefficient of self-induction of B ; a, and i 2 the cur- 

 rents in P and B respectively. Let e be the potential-difference 

 between the points V and V x . Now 



e=E sin at, where a= -~-> 



T being the periodic time, and E = the maximum value of e. 

 Also 



h=A. x sin (at— fa), where tan^s — > 



and 



* Communicated by the Physical Society : read March 10, 1888. 



i 2 = A 2 sin (at— fa), where tan fa— — -• 



r 2 



