Twj Tons produced in Gases hy Rontgen Radiation, 131 



The ionic velocities in different gases can be compared from 

 the curves obtained for these gases when in each case the 

 velocity of the opposing blast used is the same. 



In determining the numerical value of the ratio of the two 



U V 



ionic velocities in any gas from the relation — = ..== it is 



necessary not only to find two voltages for which the quantities 

 of electrification that reach the gauze in a given time are the 

 same for the two cases, bnt also to determine what in these 

 cases are the effective potential gradients producing the 

 motion against the gas-stream. 



The consideration of the gradients existing in the apparatus 

 used under these conditions is deferred to § 11, where the 

 subject of potential gradients in gas conduction is treated. 

 It is there shown that although the actual gradients differ 

 considerably from a uniform gradient between the plates, yet 

 for the two cases in question the ratio of the effective gradients 

 is not much different from the ratio of the voltages used on 

 the charged plate. 



The difference that exists between these two ratios has been 

 treated as a correction which has to be added to the ratio of 

 the two voltages used, and which amounts to about 2 per cent, 

 for the case of air. 



In using curves like those shown in fig. 4 for obtaining 

 the ratio of the voltages required to give the same deflexion 

 with the two kinds of ions respectively, the ratio is simply 

 taken of the abscissae corresponding to two points on the 

 curves which have equal ordinates, as is indicated by the 

 dotted lines. 



The place on the curves that is used must be above the 

 turning-point B of the curves, but must not be so high as to 

 be near the turn in the curves which appears when the 

 saturation value of the electric current is approached. In 

 practice the parts of the curves in the neighbourhood of the 

 letter C were used, as deemed most favourable for accuracy. 

 However, in making a determination the values are not best 

 obtained from complete curves. 



The curves are found simply as a preliminary part of the 

 operation, so as to be able to tell what are the proper potentials 

 to employ for the particular gas and blast in question. 

 Suppose that in the case shown in fig. 4 it is decided to use 

 — 60 volts for the potential of the charged plate when the 

 negative ions are dealt with ; it is seen that to obtain an 

 equal deflexion with the positive ions a voltage between + 72 

 volts and -I- 74 volts must be used. To determine the exact 

 value readings are taken, as described, with the plate Q 



K2 



