116 BEI.l. SYSTEM Tr.CIIS'lCAL JOCRX.II. 



charge in nases than they ever did, if they had not studied it in air 

 ninet\- limes out of a himdred. and in other diatomic gases most of 

 the other ten. 



Let lis sii|)p()se thai the tiilie contains lieliliin of the extrenieK' small 

 density I ha\e just defined. 'I'hen so long as the kinetic energ\' of an 

 electron does nnl exceed lit. 7') Nolts, it will rehoimd from any helium 

 atom which it strikes, like a vcr\- small perfecth' elastic ball rebound- 

 ing from a \XTy large one. We might conceive the contents of the 

 tul)e (for this purjiose and onl\- for this purpose!) asa flock of immense 

 ivory pushballs floating languidK' about, with a blizzard of etjually 

 elastic golfballs or marbles darling through the interspiices and occa- 

 sionally striking and bouncing olT from one of the pushballs. If the 

 collisions between electrons and atoms are jjerfectly elastic, as I have 

 s;iid wilhoul giving e\idence, the electron will lose an extremely small 

 part of its kinetii energy at each collision, owing to the great disparit\- 

 in masies — a fraction varying from zero up to not more than .0()().").'i7 

 depending on the direction of rebound. 



This wah verified in a prett\- experiment by K. T. ('onipiini AWtl J. M. 

 Bcnade, who utilized a certain effect- which electrons produce wluii 

 the\' ha\'e kinetic energ\' exceeding 10.7.") volts at the moment of a 

 collision with a helium atom. For example, when the pressure of 

 helium was 4.'M mm. and the electrons were drawn from a cathode to 

 an anode 0.2().") cm. away, a \()ltage-difference of 20.2.5 (plus an un- 

 known correction) was required to produce this effect ; when the anode 

 was 0.90 cm. from the cathode the required voltage-difference was 

 23.45 (plus the same correction). The extra volts were spent in re- 

 placing the energ>- lost by the electrons in the collisions with helium 

 atoms over the extra (\.'.i mm.; they amounted to an average of .0003 

 of the electron's energy lost in each co'lision. excellenllv in agreement 

 with the assumption. 



.Now as for the transit of the eleclroii-siream fiom cathode to 

 anode, the helium atoms will simph' thin it down by intercepting 

 some of the electrons and turning their courses backwards or aside. 

 The greater the numlier of atoms in the path, the greater the pro- 

 portion of electrons intercepted; it can easily be seen that, so long 

 as the gas is not denser than I have s[)ecified, this proportion increases 

 as an exponential function of the number of atoms between cathode 

 and anode,'' whether this mnnber be increased by introducing more 

 gas or by moving the anode farther aw.iy from the cathode. If 



' lii<-i|iiiMit i(iiii/;ilion, as (lcs(TilH.-(l hclim. 



'The pri>()ortion iiKrcases inort- slowly when dure arc- alrc-adv so many atoms l)f- 

 twifn ancMJf and rathoili' thai an cicclron is llki'ly to strike two or more on its way 



