308 Kelvin Quadrant Electrometer as Wattmeter and Voltmeter. 



The Revolving Contact Maker. 



The revolving contact maker M, fig. 2, exhibits a peculiarity 

 worth noting. It is in itself the seat of an electromotive force, as is 

 demonstrated by placing it across the electrometer Q 2 , and running 

 the machines without excitation. A deflection of 68 scale divisions, 

 corresponding to 0*45 volt, is given if the electrometer has no 

 capacity across its terminals, that is, if G is zero. A copper brush 

 gives the same effect as a steel one. As soon as G is given a sub- 

 stantial value as compared with the electrometer itself, this deflec- 

 tion disappears. * 



When actually observing potentials in the usual way, let the value of 

 G ba varied. For a given position of the contact maker the deflec- 

 tion varied, as shown in fig. 5, in which the ordinates are observed 



deflections, and the abscissa the reciprocals of the capacity of G in 

 microfarads. We see that when G has 1 microfarad capacity, the deflec- 

 tion is practically what it would be if G were oc, and with 1 micro- 

 farad the results verify with the true value. Such inductive effect is 

 certainly rendered negligible by sufficient capacity, and it is there- 

 fore wise to examine this effect when working with a given contact 

 maker, since each one may have its own peculiarities. 



The Manganin Strip. 



The manganin strip r 3 , r 4 , fig. 2, is in lengths of 5 feet, brazed 

 together. This material has altered its resistance, as shown in 



Table nr. 



