CHAPTER V 



THE MEASUREMENT OF POTENTIAL DIFFERENCE 

 AND ELECTROMOTIVE FORCE 



The most obvious method of determining the potential diffe 

 ence between two points in a circuit is to connect them throuj 

 a suitable galvanometer which is in series with a high resistant 

 The potential difference, P.D., is the product of the galvanomel 

 current and the total resistance of the galvanometer circuil 

 The galvanometer and the resistance may be combined in 



FIG. 129. Weston moving-coil voltmeter for direct-current circuit. 



single instrument and such " potential galvanometers" were 

 common use before the introduction of voltmeters or insti 

 ments where the products are taken once for all and marl 

 on the scales. The various types of instruments which have 

 been described as ammeters may be used as voltmeters, provide 

 the total resistance of the instrument be made sufficiently hij 

 by the use of proper windings and series resistances. There 

 differences of detail; for example, the resistance of the controllii 



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