= ^^ or ^0,. 



a Wave- Tracer and Analyser. 551 



honc(» tbo deflexion of tlu) o;alvjni()nioter will be the same as 

 that produced l)y a steady current 



Q 4M 



T ^^ TT 



Tf C/ contains both even and odd harmonics, it is easy to 

 show that Qt'-Q^^'^ is equal to twice the sum of the odd 



harmonics in C', in other words is equal to twice its first 

 component. In such a case the deflexion oi the galvano- 

 meter would be the same as that produced by a steady 

 current 



r-r "' 



where P]^ is the value of the first component of C at the 

 instants of commutation. 



3. The reducing factor \ of the galvanometer can be 

 determined by means of a Clark's cell and a megohm, or by 

 placing the galvanometer with a large resistance in its circuit 

 as a shunt across a standard resistance through which a con- 

 tinuous current determined by a Kelvin balance flows. The 

 latter is the preferable method, as the balance can be used to 

 measure the alternating currents dealt with, and they can 

 also be determined, as I shall show, from the square root of 

 mean square of the galvanometer deflexions, thus giving a 

 valuable check on the accuracy of the method. 



X having been determined^ if <y be the steady galvanometer 

 deflexion in § 2, 



- 4M„ 



by which the instantaneous value of the primary current at 

 time t is given absolutely in terms of the galvanometer 

 deflexion. 



Also if 7 be the square root of mean square of the galvano- 

 meter deflexions taken at suitable equal intervals over half a 

 wave, then the primary current in virtual units is given by 



XrT- 

 4M ^ 



4. The type of commutator used is the well-known one 

 illustrated in fig. 1 (PI. XXH.). This one is suitable for a 



2 2 



