with Mercury contained in Tubes. 



527 



Pressure. 



Current. 



Volts per cm. 



Yolts per cm. 

 sq. rt. of Press. 



25 



•035 



06-5 



13-3 



30 



•033 



73 



13-3 



41 



•031 



84-5 



132 



GO 



•028 



1075 



139 



80 



•028 



1225 



13-8 



100 



•028 



139-0 



139 



resistance, and therefore to the currents, are given in 

 the above table. The mean electric force in the arc for a 

 constant current is then directly as the square root of the 

 pressure. 



If the volts on the arc are plotted against the arc length 

 for a given pressure, and for a fixed electromotive force, the 

 result is a straight line. This line generally cuts the axis of 

 volts above the origin, indicating the presence of an anode 

 and cathode drop of potential of a few volts. If, however, 

 the results are plotted for different electromotive forces, this 

 effect is entirely masked and a straight line through the origin 

 gives the relation between the difference of potential and arc 

 length for any pressure. A diagram of this kind is given in 

 fig. 17, in which the full lines show the range of each set of 

 experiments at each pressure. 



These lines could not be extended by experiment towards 

 the origin, as the arc would not light on low voltages. This 

 difficulty has already been mentioned in describing fig. 15. 



The volts on the lamp for a given arc length can be read 

 from fig. 17. Thus, for arc length 3 cm. we have : — 



Pressure. 



Volts on 

 Lamp. 



Volts 

 Boot Press. 



25 



197 



39-4 



30 



218 



40-5 



41 



254 



397 



60 



318 



426 



80 



379 



42-4 



100 



417 



417 



which gives us that the mean electric force is approximately 

 as the square root of the pressure for the same arc length. 



