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CXIX. The abnormally long Free Paths of Electrons in 

 Argon. By J. S. Townsend, M.A., F.R.S., Wykeham 

 Professor of Physics, Oxford, and V. A. Bailey, M.A., 

 Queen's College, Oxford*. 



IN the March number of the Philosophical Magazine 

 we gave the results of experiments on the motion 

 of electrons in pure argon which showed that the mean 

 free path of the electrons is much greater in argon than 

 in nitrogen or hydrogen, and that the loss of energy of 

 an electron due to a collision is much less in argon than 

 in these two gases, for velocities of agitation of the 

 electrons between 8*5 X 10 7 and 14 X 10 7 cm. per second. 



Within this range of velocities the free path in argon 

 increases as the velocity of agitation diminishes, and it is of 

 interest to find whether the free path continues to increase 

 when further reductions are made in the velocity. 



With the same electric force and gas pressure the velocity 

 of agitation of the electrons is much higher in argon than 

 in the other gases, and it would be necessary to prepare a 

 very large quantity of pure argon or to use extremely small 

 electric forces in order to obtain small velocities of agitation 

 in the pure gas. It is undesirable, for experimental reasons, 

 to adopt either of these methods. 



The velocity of agitation may, however, be reduced to very 

 low values by adding hydrogen to the argon, and the mean 

 free paths of the electrons may be found for the mixture by 

 determining the velocity in the direction of the electric force 

 and the velocity of agitation, by the method we have already 

 described. The mean free path in argon may thus be found 

 in terms of the mean free path in hydrogen. 



Experiments on this principle were made with an improved 

 apparatus which was first carefully tested with pure hydrogen 

 admitted through a palladium tube. The velocities in hy- 

 drogen which were obtained, agreed with those already given 

 within experimental errors which may be taken as not 

 exceeding 3 per cent. 



The results of some experiments with the mixture are 

 given in the following table, Z being the electric force in 

 volts per centimetre, W* the velocity in cms. per second 

 in the direction of the electric force, an the velocity of 

 agitation of electrons in pure hydrogen at 5*1 millimetres 

 pressure, W m and u m the corresponding velocities with the 

 same electric force in the mixture consisting of hydrogen 

 * Communicated by the Authors. 



