1909] THE CORONAS OF CLOUDY CONDENSATION. 179 



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,^ etc., 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 com- 

 pletely so that a method of correction appears in the work below. 

 Even when the electric lighting circuit was completely cut out, the 

 electrostatic drift in question remained. 



The measurements were standardized and the electric system 

 charged by a Carhart-Clarke cell. 



The radium tubelets used were as follows : 



No. I, 100 milligrams, strength 10,000 X 

 No. II, 10 milligrams, strength 200,000 X 

 No. Ill, 100 milligrams, strength 10,000 X 

 No. IV, 100 milligrams, strength 7,000 X 

 No. V, 100 milligrams, strength 20,000 X 



3. 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, C, C", Fig. i, are needed. It 

 suffices, however, to measure three capacities, viz., 



1. 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, 



3. That of a standard condenser, C, for reference. 

 In the present paper C was computed by the equation 



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



"The disposition of condensers C, C", cell, etc., earthed at e is sug- 

 gested in Fig. I. 



