THORIUM IN AN ELECTRIC FIELD. — HENDERSON. 5 



The first line gives the conditions under which the ex- 

 posure took phice. :"'he first column shows the electrode 

 measured. The column marked "Period" gives the time 

 taken by the gold leaf in moving over the scale divisions in- 

 dicated in the next column. The column marked "Time" 

 shows the time of conclusion of each measurement. The 

 next column shows the rate of fall of the needle, while the 

 last column shows this rate corrected for natural leak, and 

 the decay when necessary. The corrected rate is a measure 

 of the activity of the plate. The percentage cathode act- 

 ivity 



_ Cathode activity X 100. 

 Cathode activit r + anode activity. 



The error in the values of the percentage cathode activity 

 was mainly that made in measuring the anode activity. This 

 error was seldom as much as 2 per cent. The readings were 

 usually repeated several times and the mean taken. 



An objection to these vessels might be raised, namaly, 

 that the plates are very close together at the edges, and that 

 this distorted field was the determining part of the field. 

 That this was not the case was shown as follows: 



An exposure was taken as usual. The activities of the 

 plates were measured as usual. Then the plates were covered 

 with a zinc screen neatly covering the whole of the plates 

 except a part in the flat portion of the plate where the screen 

 was cut out, and which could expose a portion of either the 

 top or bottom half of the plate. The size of this opening 

 was 5.8 X 2.8 cm. 



The activities of the plates were then measured with the 

 lower and upper portions exposed. he results follow: Box 

 6. Plates 15. Volts 2.1. Laboratory air. 



