Electricity carried by a Gaseous Ion. 351 



the cloud-chamber was lined with damped black silk, a narrow 

 slit being left to allow the cloud to be observed. The bottom 

 of the chamber was a plane piece of glass, and the fog was 

 illuminated by a vertical beam of light reflected from a 

 mirror. The top of the cloud-chamber was made of sheet 

 aluminium *3 mm. thick, this allowed the rays from the 

 radium to pass through. Two different samples of radium 

 were used : the one called B was placed at a distance of 

 15 cms. above the top of the cloud-chamber, the other (a 

 weaker specimen), A, was 10 cms. above it. The radium was 

 spread over surfaces measuring 7 cms. by 5 cms., and was kept 

 in its place by a thin sheet of mica. 



The electrical part of the apparatus included a guard-ring 

 condenser, the upper plate of which was a circular piece of 

 aluminium of the same thickness and cut from the same 

 plate as that forming the top of the cloud-chamber ; the 

 lower part of the condenser consisted of a circular plate of 

 aluminium 2*47 cms. in radius in one piece of apparatus, 4*55 

 in another, surrounded by a guard-ring also of aluminium ; 

 the distance between the upper and lower plates of the con- 

 denser was 1 cm. The radium was placed above the upper 

 plate of the condenser and at the same distance from it as in 

 the cloud experiments, i. e. the sample A was placed 10 cms. 

 and the sample B 15 cms. above the plate. The lower cir- 

 cular plate was connected with one pair of quadrants of a 

 Dolezalek electrometer, the wire making the connexion being 

 led through a metal tube connected with the earth. The 

 radium was surrounded by lead guards so as to confine its 

 radiation to the neighbourhood of the condenser. The radium, 

 the guard-ring condenser, and a second condenser used to 

 determine the capacity of the system were placed inside a 

 box lined with metal. The top plate of the condenser was 

 maintained at a constant potential ; the lower plate, which 

 was in connexion with the electrometer, was initially put to 

 earth. When the earth connexion was then broken, a current 

 of electricity passed between the plates through the air ionized 

 by the radium; this current charged up the electrometer, and 

 the needle was deflected. Knowing the deflexion of the elec- 

 trometer produced in a given time, the quantity of electricity 

 received by the lower plate in that time can be calculated, if 

 we know the capacity of the electrometer and its connexions. 



If u is the mean velocity of the positive and negative ions 

 under the electric field applied to the air between the plates. 

 a the number of ions (positive and negative) per c.c. of the 

 air, e the charge on an ion, A the area of the lower plate. 

 the quantity of electricity received by the lower plate in unit 



