THE MERCURY ARC 95 



nous column. With pressures greater than one millimeter 

 fairly accurate measurements can be made by reading the 

 difference in level between the two arms of a manometer, 

 but with pressures less than this such readings have serious 

 errors and these lower pressures are important ones in 

 forming a theory of the arc. 



Wills used the following indirect method of finding the 

 vapor pressure. The temperature was first determined by 

 the method already described and from this, by using tables 

 which give the relation between the pressure of mercury 

 vapor and its temperature, the pressure was computed. 

 This assumed that the thermojunction gave the correct 

 value of the temperature and also that the pressure in the 

 arc is the same as it would be, if the vapor and the liquid 

 in contact with it were at the same temperature, which is 

 the condition present when measurements are made on 

 vapor pressure. The first of these assumptions has already 

 been criticized and the second is certainly incorrect. 

 There can be no question but that the vapor through 

 which the current is passing is very much hotter than the 

 liquid, which is only a little warmer than the surrounding 

 air. 



However, some idea of the relation between voltage and 

 current can be had from the curve given by Cooper Hewitt, 1 

 which is shown in Fig. 28. The circles on the continuous 

 curve indicate the final voltages between the terminals of a 

 mercury lamp with different currents. The crosses indicate 

 readings which were taken immediately after the current 

 had been increased and before the pressure of the gas had 

 changed to any great extent. Thus the cross at a indicates 

 the reading when the current had suddenly been increased 



1 Elec. World, 37, 679; 1901. 



