112 DE. W. G. DUFFIELD, MESSES. THOS. H. BUENHAM AND A. H. DAVIS ON 



Method 2. The disposition shown in fig. 3 was employed, the fixed carbon rod 

 occupied either position A or B. When in position A the arc was vertical and the 

 couple was caused by the electromagnetic action between the movable part of the 



B 



Fig. 3. 



apparatus and the rest of the circuit. When placed in position B the arc was 

 horizontal and there was an additional couple occasioned by the pressure upon the 

 pole. The difference between the readings gave the couple to be measured and 

 hence the pressure in dynes. The results for currents of 9 and 12 amperes are 

 recorded in row 2 of Table I. 



Method 3 (Double Arc). The mercury cup was removed and the circuit completed 

 through a second arc shown at C or C' in fig. 4 ; as it was vertical it did not add 

 anything to the deflecting couple upon the copper rod. It constituted an extremely 



1C' 



Fig. 4. 



flexible electrical joint. As before, A and B were alternative positions for the other 

 carbon, in the latter of which the arc was horizontal and the pressure effective ; the 

 differences between A and B measured in a typical experiment are recorded in 



