Fiar. 1. 



/WA 



I 



552 Dr. A. S. Eve on the Number of Ions produced by 



numbers, necessarily speculative in their values, have been 

 estimated for the 7 rays. 



Electroscope. 



The electroscope was formed of a light framework of 

 knitting-needles, standing on wooden 

 posts (fig. 1). The framework was 

 covered with aluminium leaf, *003 cm. 

 thick, offering a slight resistance to the 

 /3 particle, not exceeding that of a cm. 

 of air. The internal measurements were 

 22 x 11-5 x 11*5 = 2910 cm. 3 The central 

 axis and aluminium leaf were insulated 

 by amber beads, above and below, and 

 the axis passed through the upper bead 

 into the air for a distance of 7 mm., and 

 this exposed end was covered with an 

 ordinary thimble, which could be re- 

 moved temporarily for charging. The 

 usual charging-key was thus unnecessary 

 — a great advantage when determining 

 the capacity. One division of the micro- 

 scope scale corresponded to 2*65 volts. 



Capacity. 



It is difficult to determine with accuracy 

 a small capacity, such as 2 or 3 cm. 

 A method is given by Harms in the Pliys. Zeit. (Jan. 15, 

 1904), and another by Campbell (Phil. Mao-. Jan. 1911), but 

 these involve, to some extent, connecting wires. 



I have found it possible to obtain the capacity by taking 

 straight thin wires, of length I and radius r f the capacities 

 of which, away from conductors, are given by 



/ 



2 log, 



V 



Two such wires, each of length 20 cm., having separately a 

 capacity of 1*82, when placed end to end, together have 

 a capacity of 3' 21. When placing a vertical wire of length 

 20 cm. in contact with the axis of the electroscope, it was 

 regarded as approximately one-half of a wire of 40 cm. 

 length. There are further corrections, such as that due to 

 the approach of the wire to the earth-connected upper plane 



