486 Mr. A. B. Meservey on the Potentials required 



diameters. The positive and negative curves therefore make 

 larger and larger angles with each other and with the 

 parallel plate curve as the diameter decreases. Figs. 6 and 

 7 show similar curves (a and a' for positive, b and V for 

 negative inner electrodes, c and c' for parallel plates) for 

 several different pressures in air, and exhibit a progressive 

 change in the forms and relative positions of the different 

 carves. As the pressure becomes less, the positive curve 

 lies higher as a whole with reference to the parallel plate 

 curve ; while in the negative curve the divergencefrom the 

 positive becomes less, and finally, instead of bending away 

 from the positive, the negative actually rises and approaches 

 it. The rise in the sparking potential for large diameters 

 with negative inner electrodes at pressures of *5 and *3 mm., 

 is due to the fact that for these diameters the pressure is 

 below the critical value, so that the pressure-sparking poten- 

 tial curve is rising very rapidly. The curves for hydrogen, 

 of which two pairs are shown in fig. 8, exhibit similar 

 general characteristics, but are somewhat more confusing 

 owing to the fact that the critical pressure is higher in 

 hydrogen than in air, and the rise for the large diameters is 

 much more marked for certain pressures. 



In fig. 9 the diameters of the inner cylinder are taken as 



Fig-. 9. 



3£ 



li 2.0 i-4 18 



Diameter in M, m , 



32 



abscissae, and the corresponding values of the minimum 

 sparking potential as ordinates. For the largest diameter 

 employed the minimum sparking potential in air is 380 volts 

 with the inner cylinder positive, or about 30 volts above the 

 parallel plate value ; when the inner cylinder is negative, 

 the minimum sparking potential is 330 volts, or about 

 90 volts below the parallel plate value. As the diameter is 



