8 L. G. Raub 



Certain systematic errors arise in measuring the distance from 

 the center of K to the probe. One is due to the impossibihty of 

 determining when the probe passes through the axis of the cathode. 

 This was obtained as accurately as possible by drawing K back 

 until the probe would rotate past the end and adjusting 6" while 

 the probe-wire was held at the center of the cathode. A second 

 error arises from the impossibility of making the probe exactly 

 straight and exactly parallel to the axis of the cathode. These 

 errors were made sufficiently small to be negligible except at points 

 very near the cathode surface. However, the deviations found at 

 the edge of the polarization region are consistent and would seem 

 to arise from these errors. 



A third, error which is not systematic and has a greater effect 

 upon the computed results arises from the difficulty in locating the 

 point of minimum gradient, r,„ of equation (8). This value, 

 which has a great influence on the computed potentials, is not well 

 marked by the luminosity in the gases used. It was determined, 

 approximately, by taking several readings in this region and plot- 

 ting observed potentials against distances, or radii of the equi- 

 potential surfaces. The most probable values of r^, and V^ were 

 then taken from this curve and substituted in equation (8), to- 

 gether with the values of two or three other equipotential surfaces 

 not too near the negative glow. The value of the factor 1.34 X 10^ 

 (I^/U)^^ was then computed, and those values of r^ and Vq 

 which gave most nearly a constant value for this factor were taken 

 as corresponding to the point of minimum gradient. The average 

 of the values obtained by the above computation for the factor 

 1.34 X lO*' (I^/U) '2 was then used in computing the values of V 

 recorded in the tables. 



The current was furnished by a battery of small lead storage 

 cells and measured by a Weston milliameter (Fig. 2). The dif- 

 ference of potential between the cathode and the probe was meas- 

 ured by a well-insulated quadrant electrometer of the Dolazalek 

 type with a phosphor-bronze needle suspension. The cathode was 

 an aluminium wire carefully cleaned and polished. 



The apparatus for generating and purifying helium is shown in 

 Fig, 3. Dry, powdered clevite was placed in flask A^ and granular 



