594 Prof. F. Horton and Miss A. C. Davies 



on 



difference Y 4 , between the gauzes B and C, was in the- 

 reverse direction to Y 1 and V 2 , and was sufficiently great to- 

 prevent any of the electrons from reaching B. The usual 

 path of the electron stream is thus indicated by the arrow in 

 the diagram. A represents the collecting electrode which 

 was connected to a sensitive electrometer for measuring the 

 currents due to ionization or radiation. Its distance from 

 the gauze B could be varied over a range of several centi- 

 metres. Inmost of the experiments the apparatus employed 

 was that which we had used in our previous investigation 

 with helium, but the gauze C had been added, the distance 

 between C and D being about 1 cm. The later experiments 

 were made with a slightly modified form of apparatus in 

 which the distance between C and D was about 3 cm., and 

 this apparatus was afterwards used for experiments with 

 argon ; a complete description of it is given in the Pro- 

 ceedings of the Royal Society, A. vol. xcvii. p. 1. The 

 helium used was carefully purified before being stored, and 

 during the observations was slowly circulating through the 

 experimental tube, entering through a U-tube containing 

 carbon and' immersed in liquid air. The pressure of the gas 

 during the experiments was varied according to the particular 

 point which was under investigation. 



In considering the points at which breaks might be 

 expected in the current-potential difference curves for high 

 values of the accelerating potential difference, we started 

 from the facts already established, that radiation is produced 

 from helium by impacts of e'ectrons with 20*4 volts velocity, 

 and ionization by impacts of electrons with 25'6 volts 

 velocity. Under suitable conditions of pressure it should be 

 possible to obtain curves giving indications of the production 

 of radiation and of ionization by the second, third, etc. 

 inelastic collisions of electrons with helium atoms. The 

 particular points on the curve at which indications are to be 

 expected will depend upon whether the distance over which 

 the accelerating potential difference is applied is large or 

 small compared with the mean free path of an electron in 

 the gas. For instance, in order that a radiation curve 

 should show increases of radiation at multiples of the 

 resonance velocity (20 4 vojts), the pressure must be so high 

 that most of the electrons in the stream from the filament 

 make several collisions in traversing the distance over which 

 the accelerating potential difference is applied, so that for 

 the range of velocities used the potential fall along a free 

 path is not likely to exceed the resonance voltage. Under 

 these conditions, very little, if any, ionization will take 



