Currents to the Study of Ionization by Collision. 467 



platinum strip had been established by this arrangement, 

 was tested in the following way. The plate A was raised to 

 a positive potential of 4 volts and the thermionic current 

 was measured when it had become practically constant. 

 The heating current was then reversed and the thermionic 

 current again measured when it had become steady. This 

 was repeated several times and the mean of the galvanometer 

 readings for each direction of the heating current was found 

 to be the same. In this test it was necessary to take the 

 mean of several readings, as the thermionic current from 

 the lime was slowly decreasing with time. This gradual 

 decrease in the electron emission from lime is most marked 

 in the early stages of heating. After several hours' heating- 

 it becomes small, and at the temperatures used in the present 

 research the current was practically constant during the 

 course of ten minutes or so. 



The experiments consisted in measuring the currents 

 between the two plates A and B at different distances 

 apart. The plates were accurately parallel to each other. 

 This was tested by lowering the plate A until it nearly 

 touched B and then examining the width of the space 

 between them with a microscope, looking in two directions 

 at right angles. The cylinder C could be moved slightly on 

 the spring clips which held it in position, and this movement 

 allowed B to be adjusted parallel to A when examination 

 showed that such adjustment was necessary. The distance 

 apart of the plates was measured to '001 cm. by means of a 

 travelling microscope. A piece of white paper was placed 

 behind the apparatus and this was illuminated by a lamp 

 placed on the further side of it. The image of the edge of 

 the plate seen through the microscope was very sharply 

 defined, and it could be adjusted on to the horizontal cross- 

 wire with considerable accuracy. In the experiments, the 

 distance between the plates was always adjusted so as to 

 be an exact number of millimetres as measured by the 

 microscope. The apparatus (seen in fig. 1) was firmly 

 supported in a strong iron retort-stand resting on a slate 

 slab which also supported the measuring microscope, so that 

 no error was introduced through relative motion of the two 

 supports. 



The measurements were taken in the following order : — 

 The distance apart of the plates was first adjusted to 1 mm., 

 and a difference of potential V was established between 

 them. When the thermionic current was steady, the reading 

 of the galvanometer G was observed. The travelling micro- 

 scope was then raised 1 mm. by means of its micrometer 



