446 Mr. S. Ratner on the 



air passes through calcium chloride, concentrated sulphuric 

 acid, and glass-wool. The plate A may be earthed or charged 

 to a desired potential by means of a battery o£ small cells^ 

 one terminal of which is earthed, while the plate B is con- 

 nected to a galvanometer provided with a series of shunts, 

 by means of which any current between 10~ 9 and 10~ 5 

 ampere could be measured. The strip of platinum, coated 

 with a mixture of barium oxide and aluminium phosphate, 

 emits when heated a copious supply of negative or positive 

 ions, according to the direction of the electric field between 

 the plates. The motion of these ions, as shown above, 

 produces between the plates a stream of gas which passes 

 by inertia through the grating gh and a small tube t, and 

 imparts to the vane a deviation measured in the ordinary 

 way by means of the mirror r. 



When the platinum strip is heated, no electric force being 

 applied between the plates, a convection current due to the 

 heating of the strip is produced in the gas, and the image 

 from the mirror changes its zero position on the scale. For 

 each temperature of the strip a correction of the zero-point 

 of the apparatus is therefore necessary, and precautions must 

 be taken in order to diminish, as far as possible, these dis- 

 placements of the zero. The suspension of the vane should 

 not be too sensitive, the platinum strip has to be very narrow 

 and placed in a vertical line, as shown in the figure, and the 

 vane should not be placed close to the edge of the tube £, 

 but about 5 millimetres from it. The ionization current, as 

 well as the corresponding wind-pressure, may be varied 

 over a wide range by changing the temperature of the strip, 

 or the electric force between the plates. 



It is seen from equation (1) that for a given distance d 



to 

 between the plates the mobility of an ion ^ is proportional 



c '. . 



to p, the ratio between the ionization current and the 



corresponding wind-pressure. A great number of preliminary 

 experiments were carried out in order to verify the above 

 equation. As at high pressures the mobility of an ion is 

 known to be constant over a wide range of the strength of 



the field, the ratio p, according to equation (1), must also 



be constant under the same conditions. Further, supposing 



-— =&(_) for negative ions and p=k {+) for positive, -~^ 



must be the ratio between the mobilities of the negative and 

 the positive ion. 



