L891.] Power given by any Electric Current to any Circuit. 425 



tiu 



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urrent to the primary coil of a transformer. His original proof, a 

 geometrical one, was based on various hypotheses, amongst others, 

 that the primary and secondary currents and the magnetic flux were 

 ine functions of the time. 



Recently, one of us, in conjunction with Mr. Taylor, has published* 

 analytical proof showing that Mr. Blakesley's three dynamometer 

 method of measuring power gives equally true results, whatever 

 functions the currents and magnetic flux be of the time. There still 

 owever, remains a serious objection to this method, viz., that it 

 umes the absence of magnetic leakage in the transformer, or in 

 ther words, that the number of lines of force embraced by one con- 

 olution of the primary coil at any moment is the same as the number 

 f lines of force embraced by one convolution of the secondary, 

 iher, the three dynamometer method cannot be used to measure 

 e power given to a single circuit, as the coils of one of the dynamo- 

 eters have necessarily to be put in different circuits. 

 The employment of an electromagnetic wattmeter for the measure- 

 nt of electric power is well known, and investigators have con- 

 idered the error that is introduced into wattmeter measurements 

 de with alternating currents on account of the fine-wire circuit 

 f the wattmeter possessing self-induction. This fine-wire circuit 

 lually consists of a suspended coil in series with a so-called non- 

 ductive stationary high resistance, and various devices have been 

 opted by different experimenters to make the effective self-induction 

 I this fine-wire circuit nought. One of the simplest of these devices 

 e venture to think is that proposed by one of us in conjunction with 

 T. Mather, and which consists in winding the stationary so-called 

 on-inductive resistance in such a way that the capacity of this 

 iubly-wound coil practically neutralises the effect of the self-induc- 

 ion of the suspended coil. 



II. 



Several months ago, however, while working at alternate current 

 iterference, we noticed that it was possible to employ an extremely 

 iple method, based on the difference of phase of the P.D. and the 

 irrent, for measuring the power by any current to any circuit. This 

 lethod, which has since been in regular use in the laboratories of the 

 Central Institution, is quite independent of any assumptions as to the 

 iture of the current, or of the circuit, the power given to which it is 

 lesired to measure, and it has the further great advantage that the 

 )nly measuring instrument required is the ordinary alternate-current 

 voltmeter of commerce. 



In series with the circuit ab (fig. 1), the power given to which we 

 lesire to measure, connect a non-inductive resistance be of r ohms. 

 * Meeting of Physical Society, February 27, 1891. 



