OP ARTS AND SCIENCES. 



137 



XI. 



CONTRIBUTIOXS FROM THE PHYSICAL LABORATORY OF 



HARVARD COLLEGE. 



NO. XIV. — ON A NEW METHOD OF COMPARING THE ELECTRO- 

 MOTIVE FORCES OF TWO BATTERIES AND MEASURING 

 THEIR INTERNAL RESISTANCE. 



By B. O. Peirce, Jr. 



Presented, March 14, 1877. 



The relative strengths of two currents which give ri?e to different 

 deflections of a galvanometer needle are not easily compared, and 

 therefore it has been the aim of pliysicists in making electric meas- 

 urements to choose such methods as require the current through the 

 galvanometer to be made either zero, or else equal in two given cases. 

 The method of measuring resistances by Wheatstone's Bridge is an 

 example of a '' nul" method. Mance's and Thomson's methods of 

 measuring the resistances of batteries are examples of the class which 

 require equal deflections. 



Poggendorff's method of comparing the electromotive forces of 

 two batteries is the only accurate method vehich belongs under either 

 of these heads. 



I have found that very accurate results can be obtained in the 

 following way : — 



Fig. 1. 



The batteries to be measured are arranged as in the figure, with 

 their electromotive forces conspiring, r is any small resistance. R^ 



