Electroscopic Measurements and their Prevention. 27U 



avoided in the construction of gold-leaf electroscopes, and as 

 some of the effects obtained are interesting in themselves, 

 a short account of them may prove useful. 



In a cylindrical electroscope of the ordinary type con- 

 taining a gold-leaf system insulated by a sulphur rod, if the 

 time for the leaf to cross the scale in the eyepiece of the 

 microscope was taken and the experiment repeated a number 

 of times, the rate of leak gradually increased until a maximum 

 rate of leak was established. This effect was obtained with 

 both radium and uranium X as the ionizing source and 

 with both (3 and 7 rays. The effect was obtained also, when 

 the leaf was charged continuously either negatively or 

 positively and in C0 2 as well as in air. The effect was 

 shown to depend on the presence or absence of a charge on 

 the leaf system ; for if the charge and the ionization were 

 maintained, the rate of leak tended towards a maximum ; 

 while if the ionization was maintained for a sufficient period 

 after the charge had been dissipated, the rate of leak became 

 a minimum. 



In the initial observations of this varying-leak effect, the 

 gold leaf used throughout had been charged to a potential 

 rather higher than advisable. The fact that the cause of the 

 effect might possibly be due to a high potential of the leaf, 

 suggested the weighting of an ordinary gold leaf by a short 

 piece of '025 mm. platinum wire. By doing this a much 

 higher potential is obtained for a given divergence of the 

 leaf than with an ordinary gold leaf. The weighted leaf was 

 found to exaggerate the effect previously obtained and to 

 bring to light others which, by their smallness, had not been 

 brought out before. 



Fig. 1 (p. 280) is a curve which shows graphically some of the 

 effects obtained. The rate of discharge of the weighted leaf 

 in scale- divisions per minute as ordinate, is plotted against 

 the time at the commencement of each leak from the start 

 of the experiment, as abscissa. 



The electroscope had not been used for some time and the 

 leaf was discharged. The leaf was charged up negatively 

 by means of a charging rod, and the rate of discharge of 

 the leaf under the 7 rays of the constant ionizing source 

 measured. A represents the leak in scale- divisions per 

 minute corrected for natural leak. The leaf was imme- 

 diately recharged to the same negative potential and the 

 rate of discharge measured again. Continuation of this 

 process gave the values A 2 , A ;; , A 4 , etc., each being larger 

 than the one before, until successive chargings gave no 

 further increase. B represents the leak at this point which 



