230 0. C. Lester — Emanation Electroscopes . 



Table III shows that the ratio of the activity to the amount 

 of emanation is approximately constant at a given pressure, at 

 least for the range of activity here examined. There is good 

 reason to suspect an error in the weight for the second sample. 



The maximum activity multiplied by the " constant " of the 

 electroscope and divided by the volume of water or gas taken 

 gives the number of curies per unit volume which is a fixed 

 quantity. However, since the maximum varies with the pres- 

 sure the " constant-' must vary also, but we should always have 

 activity X constant = curies or 



mk =: C (1) 



which is the familiar equilateral hyperbola or Boyle's Law 

 equation. 



Now the constants of each chamber are known accurately for 

 a temperature of 22° C and a pressure of 62*5'="\ From the 

 curves of iig. 2 we find the corresponding mean maximum 

 activities per milligram of pitchblende to be 0"881 divisions 

 per minute for chamber No. 2 and 1*077 division per m.inute 

 for chamber No. 3. Hence the constant Kp for any pressure^ 

 is found from 



KpMp = 2-053 X 10-'" for chamber No. 2. 

 and KpMp = 2-036 X 10"'° " " " 3. (2) 



Where Mp denotes the mean maximum activity per milli- 

 gram at the given pressure. 



The constant-pressure curve for chamber No. 2, shown in 

 fig. 3, is obtained from the above equation. The values of M 

 are the mean values from Table III. It is evident that even 

 the daily variation in barometric pressure is often suflicient to 

 make a decided difference in the value of the " Constant." 



Strictly speaking the curve of fig. 3 gives the constant at 

 various pressures for a temperature of 22° C. Changes in tem- 

 perature will effect the constant also in so far as they affect 

 the density of the gas in the ionization chamber. But since a 

 change of about one per cent only is produced by a change of 

 3° C in temperature this source of error may usually be neg- 

 lected. 



In the actual work of testing waters and gases a common 

 practice is to run the observations for a short time and then 

 calculate the maximum activity. Tests on such vessels as those 

 used in our work fully justify this procedure. From the curves 

 of a number of calibration tests run for three and a half hours 

 the activities were read at ten minute intervals from 10 min. 

 to 90 min. These values expressed as percentages of the 

 maximum formed always a closely agreeing scale, for a given 

 chamber, over a wide range of activity. For example, the 



