124 DR. W. G. DTTFFIELD, MESSRS. THOS. H. BURNHAM AND A. H. DAVIS ON 







whence 



At 8 amperes. 



o o o 



V = 175, 190, 178 Set A-j 



} Moan 17y 

 120, 186, 197 SetBJ 



E = 58, 60, 55 Set A 1 



^ Mean 75 

 93, 164, 22 Set B J 



At 9 amperes. 

 V = 188, 181, 181 Mean 183 



E = 94, 101, 97 Mean 97 



P= 55, 30 Set A I ,. P = 47, 48 Mean 47 "5 



., }> Mean 38 

 22, 44 SetBJ 



The couples due to P, E, and V are thus approximately as 1 : 2': 4 in the actual 

 arrangement employed, but this is accidental. The pressures, P, at 8 and 9 ampferes 

 reduce to 0'91 and 1'03 dynes respectively, values in good agreement with those 

 obtained by other methods. 



The Electromagnetic Effect. 



In order to evaluate the couple upon the suspended arm due to the electromagnetic 

 effect of the rest of the circuit, and particularly the effect of the fixed pole at right 

 angles to it, the apparatus was arranged as in fig. 17, I>. The current was led to the 

 centre of the copper rod from the mercury trough and from the fixed carbon by a 

 vertical wire. The couple was measured for different lengths of the fixed pole and 

 was found to reach a maximum at about 11 cm. Alternating current obviated any 

 influence of the earth's magnetic field. The effect of altering the current strength 

 with a fixed length of pole was also examined and the couple found to vary, as 

 was expected, with the square of the current. These data enabled the necessary 

 corrections to be made where method No. 1 was adopted (loc. cit.). 



Electrostatic Effects. 



Previous to striking the arc the electrostatic attractions between the poles 

 amounted to (V125, (T03L O'OOS, O'OOG dynes for arcs of 1, 2, 3, 4, and 5 mm. 

 respectively. But when the arc is struck the distribution of charges within it 

 entirely alters these conditions. 



In Part II. is developed a theory of the pressure which does not appear to be 

 seriously affected by electric effects within the vapour. This view is supported by 

 the observation that the pressure depends upon the nature of the poles. 



Connexion Currents. 



Hitherto we have dealt with the total pressure upon the poles, it remains to 

 consider to what extent it is caused by convexion currents of hot air and vapour 

 rising from the arc. It will appear that convexion currents tend to cause the poles 

 to move towards one another, and that if they could be eliminated the pressure 

 would be higher. 



