112 DR. W. G. DUFFIELD, MESSRS. THOS. H. BURNHAM 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 
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 difierence 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 
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 eflective ; the 
differences between A and B measured in a typical experiment are recorded in 
