94 THE ELECTRIC ARC 



difficult to determine this relation. It is not possible to 

 change the current with the arc in a tube without changing 

 the pressure of the vapor through which the current is 

 passing, and a change in the pressure of the vapor will in 

 itself cause a change in the voltage required irrespective 

 of any effect which the current may have. 



Moreover, the amount of change in the vapor pressure 

 depends on several factors. If, for example, the tube 

 containing the arc is connected to a pump the vapor pro- 

 duced by the arc passes into other parts of the apparatus 

 where it is rapidly condensed and the pressure in the arc 

 does not become large. If the lamp is not connected to 

 other apparatus, the pressure amounts to several milli- 

 meters, and with quartz tubes where large currents are used 

 the pressure may become even more than one atmosphere. 



Again the pressure depends to a great extent on such 

 things as the size of the tube, the condensing chambers 

 and the rate at which heat is taken from the tube. The 

 hotter the tube becomes, the more mercury will be vapor- 

 ized and the greater the pressure of the vapor. There will, 

 therefore, be a great difference between a tube which is 

 placed in water kept at a constant temperature and a tube 

 surrounded by air. Even a difference in the thickness of 

 the walls of a tube may easily affect the rate at which heat 

 is radiated and consequently the pressure of the vapor, and 

 the size of the tube may not only affect the pressure by 

 giving more room for the vapor, but may affect it indirectly 

 by giving more chance for the heat to escape. 



Moreover it is not easy to make accurate determinations 

 of this pressure when it is low. One can not use any 

 arrangement similar to a McLeod gauge, since the vapor 

 condenses to a liquid as soon as it is taken from the lumi- 



