462 
side. A similar window should be placed on the opposite side to admit light. 
To read the amount of deflection a scale 8, 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! and L'. 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! 
can be read on the seale, 8S, 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 14-inch brass drain cocks are soldered to the emanation chamber 
to admit the emanation. 
The data of the following experiment, Table 38, carried out by two 
students using the ‘‘tin electroscope’’ and a Schmidt electroscope made by 
Spindler & Hoyer, Géttingen, will give an idea of the accuracy of this elec- 
troscope and also the accuracy of Duane’s formula. 
TABLE 3. 
Electroscope. .. ‘ é ; Pato Ah | pL ects ereetete eters Schmidt. 
Observer..... = are BSc Hote oll ie hore tetera of tee a en Dente 
Diameter of che eaier: SA ee OL, 10.8em... ‘ ..| 7.8¢em. 
ieVSroliqaeitd tin ley Oo cn ar RP eReE Gr Monee ate a ya eee sh Piss 
Volume of chamber.... er Rn RAS, AA RO a UM PAT ORAS AP .| 968 cc. 
Surface of chamber.......... Fa pene ee sete §94.sq.cm.......... ....| 586.6 sq. em. 
Capacity of electroscope.......... PT ce PU eh AY cr 0s Re Re ae ee 6.3 em. 
Observed emanation, Curies per liter, ae lope t site > _.| 206000. X1072 ............| 200000. X10-12 
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. 
