492 Mr. A. B. Meservey on the Potentiah 



'equirec 



and upon that necessary to maintain it *. These experi- 

 ments indicate that, at least £01* the potential required to 

 produce the luminous discharge, the effect at ordinary 

 temperatures is similar to the heating effect mentioned 

 above, that is, the curve for a higher temperature is higher 

 below the critical pressure and lower above it than that 

 for a lower temperature. In Brown's experiments, all the 

 points on a sparking potential curve would be found at 

 approximately the same temperature, while those on the 

 corresponding maintenance curve might be at higher 

 temperatures owing to the passage of the current. If the 

 effect of temperature change on the maintenance curve 

 were like that on the other curve, and sufficiently great, it 

 might cause the maintenance curve to cross the other and 

 run above it when the pressure was below the critical value. 

 But the effect of temperature on the maintenance curve, in 

 Earhart's experiments, does not seem to be very determinate, 

 and at any rate is not large, so it would hardly seem that we 

 could explain the peculiar relation between the two curves 

 on the ground of temperature effects. However, the ex- 

 periments were not undertaken for the purpose of settling 

 this point, and did not give definite information on it, so it 

 was decided to continue a method of direct in v estimation. 



In the present experiments the spark chamber used (see 



fig. 10) was one of those used by Brown, and the rest of the 



apparatus was the same as his, with a few additions. The 



spark chamber consisted of two aluminium disks 5 set into 



* Physical Review, September 1909. 



