[mclennan-keys; electrical CONDUCTIVITY 53 



to the boss CD. A third brass plate GH carried a polonium coated 

 plate PQ with the coated surface facing the boss CD. EF was a plate 

 of ebonite provided with a circular opening and inserted between 

 the polonium coated plate PQ and the guard plate MN and the boss CD. 

 Care was taken in putting this apparatus together to see that the boss 

 CD did not touch either the plate MN or the ebonite plate FF. The 

 diameter of the opening in EF was 1-06 cm, and the distance between 

 the boss CD and the face of the plate PQ towards it was 1 cm. This 

 apparatus was placed in a strong steel cylinder and air was pumped 

 into the latter until the pressure stood at 120 kilograms per sq. cm. 

 The plate AB was attached to an insulated wire leading through the 

 cylinder and passing on to the quadrants of a Dolazaleck electrometer. 

 The plate M was connected directly to the steel cylinder which was 

 kept joined to earth and the plate GH was connected by an insulated 

 wire passing through the cylinder to a battery of storage cells. It 

 should be added that the apparatus was put together initially without 

 the polonium-coated plate PQ in position and was found to insulate 

 perfectly when surrounded with air in the steel cylinder at 120 kilograms 

 per sq. cm. pressure. 



With the polonium-coated plate in position one had with this 

 arrangement a layer of highly ionized air close to the plate PQ. More- 

 over the field between the plate PQ and the boss CD was a uniform 

 one and the dimensions of the field through which conduction could 

 take place to the electrode CD were perfectly defined. 



When this apparatus was put together and air pumped into the 

 cylinder, it was left intact for a period of over five months and at inter- 

 vals during this time sets of readings on the current between CD and 

 PQ were taken with series of gradually increasing potentials both 

 positive and negative applied to GH. Throughout the whole of this 

 period the currents for selected fields up to about 750 volts remained 

 practically the same, which shewed, since the intensity of the radiation 

 from the polonium must have diminished by over one half in this time, 

 that the current was independent of any variations which took place 

 in the ionization layer. 



For such a case as this Rutherford * and Child f have shewn that 

 the mobilities of the ions (expressed in electrostatic units) taking part 

 in the conduction may be deduced from the relation 



/g_ 32 . TT ■ C?^ • -^ ^ ^ (;^) 



9V- 

 where /' is the current per sq. cm. cross section, d the distance and V the 

 potential difference between the electrodes. 



^Rutherford, Phys. Rev. 19U1, Vol. 13 (6) p. 321. 

 tChild, Phijs. Rev., 1901, Vol. 12 (3) p. 137. 



