66 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



rays are wholly in question. Neither emanation nor a rays escaped the 

 double thickness of aluminum. The tube it' is grasped at t by a sheath of 

 hard rubber with an annular air-space and fixed in place by a rubber 

 cork. If care be taken to keep the tube in dry air except when in use, 

 there is no conduction leakage of consequence. 



The end /, moreover, is placed in connection with a Dolezalek electro- 

 meter, by aid of a thin wire running axially within an earthed tin drain 

 pipe and away from the fog chamber, to escape the action of y rays as 

 much as possible. In fact, their combined effect does not exceed 2 per 

 cent and is determined in special measurements. 



The keys to the electrometer were all placed on pillars of hard rubber 

 and actuated by long wooden rods from a distance. So far as possible 

 the electrical wires of the room were surrounded by earthed pipes, but 

 it was not practicable to carry this out completely, so that a method of 

 correction appears in the tables below. Even when the electric-lighting 

 circuit was completely cut out the electrostatic drift in question, supposed 

 to be due to this cause, remained. It was afterwards found that the 

 source of drift was due to an imperfection in the electrometer, but that 

 it could be eliminated by commutation, as was done. 



The measurements were standardized and the electric system charged 

 by a Carhart-Clarke cell. The radium tubelets used were as follows: 

 I, 100 nig., strength 1 0,000 X ; II, iomg., strength 200,000 X ; HI, 100 

 mg., strength 10,000 X ; IV, 100 mg., strength 7,000X1 V, 100 mg., 

 strength 2o,oooX- 



56. Auxiliary electrical condensers. To give the fall of potential 

 a suitably small value relatively to the period of the damped drop of the 

 needle a number of auxiliary condensers (fig. 19), are needed. It suffices, 

 however, to measure three capacities, viz: (i) that of the cored fog 

 chamber alone, c; (2) that of a relatively large auxiliary condenser, 

 including the electrometer, the piped wires, and the fog chamber, C" -\-c\ 

 (3) that of a standard condenser C' for reference. 



In the present paper C' was computed by the equation 



A / 

 = - - , i 



(d+dln 



- , - 



47ra\ \/7cA a* d / 



where A is the area, d the distance apart, and d' the thickness of the brass 

 plates. Since A is equal 315 sq. cm., ^ = 0.082 cm., d' = 0.67 cm., 



C' = o-6(;i + o.o84) = o cm. 



This value will suffice for the present purposes, though it needs further 

 correction by comparison with a standard condenser not now at hand. 



