H. A. Bumstead — Atmospheric Radio-activity. 5 



ural logarithms of the ionization currents are plotted as ordi- 

 nates, the time in hours being the abscissse.* The curves I 

 and III again represent the air wire, and curve IV a wire 

 exposed to radium emanation, which is given for comparison. 

 The existence of a slowly decaying activity is plainly shown 

 by the curve between two and four hours. Twenty-four hours 

 later, the wire was still appreciably radio-active, but the effect 

 was too small to measure with accuracy ; it was about one- 

 fifth as active as at four hours. 



In order to determine the rate of decay of this persistent 

 activity, six lengths of wire were suspended in a wide zigzag 

 between the second story windows of two buildings, the total 

 length being 200 meters. They were allowed to sag from the 

 horizontal by different amounts, so that their electrostatic 

 fields should include as much of the surrounding air as possible. 

 The small diameter of the wire made it possible to put this 

 length into the testing vessel without difficulty. Nine hours 

 after introducing it, the current through the cylinder was 

 about six times that due to the "spontaneous ionization," of 

 the air ; and this, although small, was measurable with some 

 degree of accuracy. Five observations were made at this time, 

 and five more after an interval of 12 hours ; combining them 

 one by one, the following values of the coefficient of decay, A., 

 were obtained : 



069 



0-069 



0-066 



0-061 



0-073 



0-065 



A = 0-067 



The half-value time corresponding to this coefficient is 10+ 

 hours ; and this is so near to the rate of decay of the excited 

 activity due to thorium as to leave little doubt that the slowly 

 decaying activity on the air wire is due to thorium. It is pos- 

 sible to increase the proportion of this to the more transient 

 activity by prolonged exposure of the wire ; for the radium- 

 induced activity, an exposure of three hours is sufficient to 

 practically reach the final equilibrium value, while the induced 

 activity due to thorium will continue to increase for several 

 days. By a twelve-hours exposure of a long wire, on a fine, 



* On this diagram, radio-active substances which decay according to the 

 same law will give parallel curves ; if the law is exponential, the curve will 

 be a straight line and the slope of the line will be equal to 2, in the formula, 



I = I e — u . The zero of ordinates is a matter of indifference so that we 

 may plot log I or (log I ± const.) as may be most convenient. 



