256 MR. E. M. WELLISCH ON THE MOBILITIES OF THE 



P and P' are tubes soldered on to the upper brass covering of the chamber in order 

 to afford an entrance for the gas or vapour, and also to admit of the vessel being 

 connected to a gauge or pump. In order to keep the vessel air-tight, a rubber band, 

 with a mixture of beeswax and resin spread uniformly over its surfaces, was inserted 

 between the outer brass and aluminium coverings which, after being heated, were 

 screwed tightly together. 



Electrometer. The electrometer was of the Dolezalek pattern with a platinum 

 suspension. The needle was charged to a potential of 80 volts, which was found to 

 be the voltage most suitable for the conditions of the experiment ; with this potential, 

 which did not however correspond to the most sensitive condition, the electrometer 

 afforded a deflection per volt of 420 mm. on a scale about 1 metre distant. As a 

 matter of fact, an exact determination of the sensitiveness of the electrometer was 

 not essential in the present investigation inasmuch as it is only the sign of the 

 charge that it is necessary to know in order to determine the points of discontinuous 

 curvature referred to in section 2. 



The leads from the upper electrodes to the electrometer quadrants were all screened 

 by brass tubing kept at zero potential. 



4. Sources of Error. - A. Theoretical Assumptions. 



Duration of Rdntgen - ray Discharge. It is assumed in the theory that the 

 duration of the discharge proceeding from the Rontgen-ray bull) is small compared 

 with the time an ion takes to describe the distance between the electrodes. This 

 duration has been estimated* to be of the order 10~' second ; the times measured in 

 this experiment varied from about O'Ol to 0'03 second. 



Reversal of Potential. After the contact K has been broken, the potential of B is 

 reversed in sign ; this reversal necessarily occupies time inasmuch as B has to pass 

 through all intermediate potentials. This time is less than the product RC, where li 

 denotes the resistance (fig. 4) and C the capacity of the leads and electrode. This 

 product in the present case was certainly not greater than 10~ 4 second. 



Influence of the lonisation on the Electrostatic Field. LANGEVIN has shown that 

 in the case of uniform ionisation, owing to the distortion of the field by the presence 

 of free ions, the time taken by an ion to traverse any distance is increased in a ratio 

 numerically inferior to Q /12o-, where Q denotes the total charge of the ions of one 

 sign and cr the charge induced on the upper plate when the potential on the lower 

 plate is withdrawn. 



For this reason it is advantageous to use, wherever possible, high voltages, but if 

 the voltages were made too high, the time taken by the ion to cross the distance 

 between the electrodes might become unduly small and the working error propor- 

 tionately large. On this account it was difficult to obtain concordant results when 



* BRUNHES, .' Comptes Eendus,' vol. 130, p. 1007, 1900. 



