212 Prof. Ayrton and Dr. Sumpner on Alternate 



2. Use a wattmeter (fig. 9), and make the capacity of 



the stationary doubly-wound resistance-coil exactly 

 balance the self-induction of the suspended and the 

 stationary coils. 



3. Make the resistance of the fine-wire circuit, cd, of the 



wattmeter small. For with a given P.D. between the 

 terminals of cd the same deflexion of the measuring 

 instrument can be obtained for different values of the 

 resistance of cd if we make the number of turns in 

 the coil or coils of the measuring instrument in cd 

 proportional to the resistance cd. But the self- 

 induction of the coil or coils is proportional to the 

 square of the number of turns, and therefore pro- 

 portional to the square of r for a given deflexion of 

 the measuring instrument. Hence tan <£ can be 

 made as small as we like for a given value of p by 

 making the resistance of cd small. 



This suggests a current method of measuring the power 

 given to any circuit which is no more wasteful of power than 

 the methods shown in figures 2, 3, 5, and 7 ; and which, 

 although not so accurate as those shown in figures 3 and 5, is 

 as accurate as those shown in figures 2 and 7. The method 

 is simply to use a wattmeter (fig. 9), but having both its 

 coils made of thick wire, or, as this may be called a split 

 dynamometer, the method consists in using a split dynamo- 

 meter having one of its coils in the circuit ab (the power given 

 to which we desire to measure) and the other coil in a circuit 

 cd parallel to ab. The power will be given at once by r 

 times the reading of the instrument and with but a very 

 small error if r be small. 



y. 



Mr. Rimington has suggested a method of measuring the 

 mean value of the product (v ± — v 2 )(vc i ~ v B) O^g- 9) by 

 means of a dynamometer, each of whose coils is in circuit 

 with a high resistance, joined up as shown in figure 10. 



The objection to this method is as follows: — By making the 

 time-constants of each of the circuits of the dynamometer afg, 



