RATIO OF UNITS. 



latter electrometric observation and the discharge of the jar through 

 the galvanometer. This latter correction was quite insignificant. 



Kohlrausch and Weber never considered their experiments exact 

 to more than 2 per cent. The difference between the extreme 

 values amounts to 7 per cent. 



1131. MEASUREMENT OF AN ELECTROMOTIVE FORCE. This 

 method was first employed by Sir W. Thomson.* Consider two 

 points A and B of a conductor traversed by a permanent current. 

 The difference of electromagnetic potential E between the two 

 points, is equal to the product IR of the intensity of the current, 

 and of the resistance between them measured in the same system, 

 and its electrostatic value e may be given by an absolute electro- 

 meter (806) the plates of which are in connection with the two 

 points A and B. 



The resistance is measured by comparison with standards. 



In order to avoid the determination of H, the intensity was 

 measured by an electrodynamometer with unifilar suspension (863) 

 which gives, for a deflection 0, 



GS' 



The difference of potential of the two points is then 



"ce 



At the same time, each of the points A and B is put alternately 

 in connection with the plate of the electrometer, the other point 

 being in connection with the shade of the instrument. If F is the 

 force necessary to bring the movable disc to the fiducial point, 

 D and D' the two readings for the distance of the plates, A the 

 corrected surface of the movable plate, we have (807) 



consequently 



* Sir W. THOMSON. British Association Report, 1869. 



E R ce A 



