Electrometer as a Wattmeter and Voltmeter. 



363 



average Watts per division given by section (a) are 17-00 for all 

 angles of phase leaving out the two values deduced by aid of the cosine 

 law for angles of 30° and 60°. It will be seen that under the con- 

 ditions of section (a) the Wattmeter may be said to be verified within 

 the limits of accuracy attainable by the method of test. The wave 

 form of the unloaded alternator is given in fig. 3 and marked C ; 

 this is the wave form of potential applied to the needle in all experi- 

 ments in sections (a) and (&). A sine curve having the same maxi- 

 mum ordinate is superposed for the purpose of comparison. The 

 current curve has different wave form according to the load on the 

 alternator. For small currents it approximates to C in fig. 3. The 

 curve A, fig. 3, is the wave form for current 74 amperes, which is the 

 maximum we have employed. 



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The experiments in section (7>), Table IT, are intended to demon- 

 strate the reliability of the instrument when the potential of the 

 needle C is varied through wide limits. One would expect from the 

 curve in fig. 1, that for high potentials on the needle the Watts, per 

 division of the scale, would diminish. This is found to b e the case 

 when the potential C is raised to 1,860 volts ( ymean 2 ) for fre- 



VOL. lxii. 2 I) 



