468 Prof. F. Horton on the Application of Thermionic 



screw, and the plate A was raised until the image of its 

 lower surface was again on the cross-wire. The difference 

 of potential between the parallel plates was then increased to 

 2 V and the thermionic current was again read. The distance 

 between the plates was increased in this way 1 mm. at a time 

 up to 5 or, in a few cases, 6 mm., and the electric intensity 

 between the plates was kept constant at the value Y volts 

 per mm. at each distance. The distances were then gradually 

 decreased down to 1 mm., corresponding reductions being 

 made in the potential of the upper plate, and observations of 

 the thermionic current were made at each distance. In this 

 way any alteration in the emission from the hot lime could 

 be detected. As a rule, the currents for increasing and 

 decreasing distances were only slightly different and the 

 mean was taken for each distance. When the differences 

 were not small the set of readings was rejected. The 

 constancy of the temperature of the platinum strip was 

 ascertained from the reading of the galvanometer in the 

 thermocouple circuit, and, when necessary, adjustments 

 were made in the resistance of the heating circuit to secure 

 a constant temperature throughout the series. The time 

 required to take a complete set of readings was usually about 

 ten minutes. 



The observations recorded in this paper were all taken 

 with dry air in the apparatus. The pressure varied from 

 0*8 mm. to 10 mm. in different experiments. The electric 

 intensity between the plates was usually 410 volts per cm., 

 the lowest potential difference between the plates being 

 41 volts when they were 1 mm. apart, but in some experi- 

 ments larger electric intensities were used. It has been 

 found experimentally by Townsend that when X/p (the 

 electric intensity in volts per cm. divided by the gas pres- 

 sure in mm. of mercury) is small, /3 (the number of new 

 pairs of ions formed by one positive ion in travelling 1 cm.) 

 is very small, and the production of new ions by collisions 

 is carried on practically entirely by the negative ions. If # 

 is zero, the general equation for the current between the two 

 parallel plates reduces to 



Under these conditions the currents ?i l9 w 2 , n 3 , measured 



with distances #, 2#, 3^, between the plates, increase in 



a constant ratio, for 



rc 2 /wi = «3/tt 2 = 



