462 



side. A similar window should be placed on the opposite side to admit light. 

 To read the amount of deflection a scale S, as shown in Fig. 4, is mounted 

 on one side on or near the back window. A strip of cross-section paper stuck 

 to the glass with paraffin while hot will answer. The paraffin serves two 

 purposes: it sticks the paper and renders the scale translucent. Half way 

 between the scale and the plane of the leaf system a mirror, M, is mounted. 

 Through the front window one sees in the mirror images of the plate, P, and 

 the foil, L, at P 1 and L 1 . These images are in the same plane as the scale, 

 S, which can be viewed by looking over the mirror, M. The position of L 1 

 can be read on the scale, S, and at the end of a convenient interval of time 

 its position can be noted again. By comparing the two positions with the 

 calibration curve of the electroscope the change of potential, dV, can be 

 obtained. 



The system is charged by means of a rubber rod through the charging 

 system, C, Fig. 2. This consists of an insulated rod with a bent wire con- 

 nected so as to be in contact with the central rod while charging. While not 

 in use this rod is rotated so as to break connection with the rod and then to 

 come into metallic contact with the case of the electroscope. 



Two 34-inch brass drain cocks are soldered to the emanation chamber 

 to admit the emanation. 



The data of the following experiment, Table 3, carried out by two 

 students using the "tin electroscope" and a Schmidt electroscope made by 

 Spindler & Hoyer, Gottingen, will give an idea of the accuracy of this elec- 

 troscope and also the accuracy of Duane's formula. 



TABLE 3. 



Electroscope 



Observer 



Diameter of chamber 



Height of chamber 



Volume of chamber 



Surface of chamber 



Capacity of electroscope 



Observed emanation, Curies per liter 



"Tin" 



F. G.T.... 



10.8cm 



12.1 



1102 cc 



594 sq. cm.. 

 17.1 cm. . .. 

 206000. XlO-i 



Schmidt. 

 W. D. S. 

 7.8 cm. 

 20.3 

 968 cc. 



586.6 sq. cm. 

 6 . 3 cm . 

 200000. X10-" 



The two electroscopes were connected together and connected to a 

 vessel containing emanation and pumped causing the air and emanation to 

 pass in a circle through the three chambers until the three contained emana- 

 tion of the same density. 



