McCoy and Ashman — Urano-Uranic Oxide. 527 



the charging device, which is connected with a three-point 

 key, that keeps F earthed, except at the moment of charging. 

 G represents a pair of glass windows, through which the 

 motion of the leaf is observed by means of a micrometer 

 microscope. The door, which slides upward, is at H. I is a 

 metallic support of variable height, for the film. L is a flat 

 metal plate 18 cm square with a 2 cm hole in the center ; it is 

 supported by a pair of brass rods at two diagonal corners and 

 may be raised or lowered. When this plate, L, is placed 

 about 3 mra above the electrode, E, the electrostatic capacity of 

 the electroscope (without the condenser, C), is about five times 

 as great as it is without L.* 



The standard capacity, C, consists of two concentric spheres 

 of sheet zinc.f The outer one is soldered to a brass block, J. 

 having a 5 mm hole, through which passes a l mm brass wire 

 soldered to the gold-leaf support; a very fine wire of spring 

 brass makes contact with the inner sphere. The latter is sup- 

 ported by three amber pins, KK ; each pin is threaded into a 

 small brass ring, so as to be adjustable ; the brass ring is carried 

 by another ring of vulcanite, which insulates it from the 

 sphere and enables the brass ring to be used as a guard ring. 

 It was found, however, that the insulation was sufficiently good 

 without the use of the guard rings. The upper half of the 

 outer sphere is detachable ; this arrangement allows the inner 

 sphere to be introduced or removed readily. The whole 

 apparatus is surrounded by a wooden case, surmounted by 

 a glass bell-jar. This serves to keep the temperature uniform 

 inside the gold-leaf chamber and so avoids air currents which 

 greatly diminish the accuracy of the activity measurements. 



The condenser spheres were made of spun sheet zinc. The 

 radii were calculated from the weight of water required to fill 

 each and the weight of the zinc of the smaller sphere. The 

 exterior radius of the smaller sphere was 6'293 cm ; the interior 

 radius of the larger 7'590 cm . The capacity of the condenser 



* We have found that the observed activity of a given film is much more 

 nearly constant when the capacity of the electroscope is increased by means 

 of the plate, L ; the reason for this is doubtless two-fold. First, the much 

 slower movement of the leaf permits greater accuracy in timing ; and sec- 

 ond, the natural variation in activity during a fixed interval is a smaller 

 fraction of the whole activity, the longer the interval. By increasing the 

 time of discharge five-fold, the fractional error due to natural variation of 

 activity would be reduced to less than half that for the more rapid discharge. 

 See Geiger, Phil. Mag., xv, 539, 1908, and Meyer and Eegener, Ann. Phys., 

 xxiv, 757, 1908. 



f The mode of combination of electroscope and condenser is a modifica- 

 tion of that suggested by Prof. Millikan ; Electricity, Sound and Light, 

 p. 351, Ginn& Co. 1907. 



