THORIUM IN AN ELECTRIC FIELD. — HENDERSON. 3 



were folded over on the dotted Hues running a sliallow box 

 one end of which was missing. Two of these electrodes, their 

 hollow sides facing; one another, were slipped open ends down- 

 wards into a wooden case. The electrodes and case set up 

 ready for an exposure are shown in cross section in Fig. 2. 

 A A are the electrodes, B B is the case, C C are wooden pegs 

 fixed in the sides of the case to separate the electrodes Che 

 edges of these were .3 cm. apart makin'^ the main parts of 

 the plates 2.8 cm. apart. D is the bottom on which the case 

 rests. It projects up a short distance inside B. The top of 

 this projection is slightl ' hollow id out, formin ; a shallow 

 trough G on which lay the thorium hydroxide, which was 

 used as source of the thorium emanation. The plates rest 

 on the rim of the trough. E E are leads and F is the cover 

 of the box. All the wooden parts of the apparatus were 

 boiled in paraffin to secure good insulation. Owing to the 

 long period of thorium B (10.6 hours) the experiments neces- 

 sarily proceeded with some slowness. Exposures varying 

 from 6 to 36 hours were made, about 24 hours being usual. 

 The plates were then removed from the vessel and their 

 activities measured. As the decay curve for thorium active 

 deposit of long exposure is practicall ' flat for the first half 

 hour, no correction for decay during the time of measure- 

 ment was needed, and even with exposures as short as 6 

 hours only a small correction was required. This has been 

 applied when necessary. The activity of the plates was 

 measured by an a ray electroscope, similar to that described 

 by Rutherford.^ I'he earthed case of the electroscope was 

 surrounded by a second case of cardboard with glass windows, 

 to protect the instrument from extraneous effects of air 

 currents and temperature. A water filter was als used to 

 eliminate the heating effect of the lamp. The gold leaf was 

 charged to 200 volts and its rate of fall vyas observed through 

 a tele-microscope. The time required to pass over a certain 



1. Rutherford, Radio-Active Substances, p. 90, fig. 12. 



