62 J. A. POLLOCK. 



2. Experimental detail,— The apparatus used in the 

 experiments, essentially a cylindrical air condenser in 

 which the leak between the electrodes is measured, is 

 similar to the instrument used by Professor Zeleny 1 in his 

 investigation of the mobility of the small ions, and to the 

 arrangement employed by M. Langevin (loc. cit.) in his 

 original determination of the mobility of the large ones. 

 It consists of a brass tube, 164 centimetres long, provided 

 with an axial electrode of the same length; the diameter 

 of the inside of the tube is 3*65, and that of the inner rod 

 0'66 centimetres. The inner electrode is divided into two 

 sections insulated from each other ; the portion at the 

 mouth of the tube, 3*7 cm. long, is earthed ; the other 

 part, 160 cm. in length, is connected by a fine wire, fitted 

 with a guard ring, to a pair of quadrants of a Dolezalek 

 electrometer, the whole of the apparatus being thoroughly 

 protected from electrification due to external causes. The 

 brass tube with its inner electrode will be referred to as 

 the testing pipe. 



Through the tube is passed a steady stream of air from 

 the compressed supply of the laboratory, which is fed by a 

 Sturtevant blower worked by a motor and storage cells. 

 The stream is measured by a gas meter, and has been in 

 these experiments of the order of 55 cubic centimetres per 

 second. In our experience the ionisation seems more uni- 

 form if the air, before being used, passes through some 

 length of tubing, and in this investigation the air travelled 

 through several metres of 2*5 centimetre iron piping before 

 reaching the testing pipe, the source of the supply being 

 the air of the laboratory workshop. 



The tube is electrified by being attached to one terminal 

 of a battery of small accumulators, the other end of the 

 cells being connected to the earthed quadrants of the elec- 



' Ztleny, Phil. Tr.ns. A., 195, p. 193, 19u0. 



