[EVE] IONS PRODUCED BY THE BETA RAYS 73 
The total number of ions per second due to a curie of radium C is, 
therefore, in air 
Orit aa. 
pe NE er ile 
He we 
On putting Q— 1, it follows that N’, the total number of ions per 
second produced in air by the y rays from one curie of radium C, is 
N’=47 X 3-74 X 10°/-000044 — 1-06 X 10” (2) 
In the work described above, the method of determining the 
capacity appeared open to criticism, although I believe that errors 
were eliminated in consequence of the number and variety of wires 
and spheres employed in the determination. 
- Professor H. A. Wilson suggested to me the use of an electroscope 
in which a dise and guard ring formed an integral part. This type of 
electroscope proved to have great merits, and it may be found service- 
able in othere ases. It is shown in Fig. 1. There are three circular 
plates of aluminium about half a millimetre thick. The upper one is 
supported by knitting needles, around which pass the cylindrical thin- 
walled sides, made of ordinary foolscap paper, lined with very thin 
aluminium foil. The knitting needles stand in ebonite posts, which 
support the other pair of circular plates. The dise was cut from the 
middle aluminium plate and suspended by a wire from an amber bead 
in the top dise. The gold leaf was fastened to the middle of the wire. 
The dise was kept in place in the guard ring by three small ebonite 
bridges, glued at each end. The lower aluminium plate was charged 
to a potential V, whilst every other part of the apparatus was earthed. 
The charge on the central wire and dise was, therefore, ie = V, where r 
T 
is the radius of the disc, and d its distance from the lowest plate. The 
dise and wire were then insulated, and all parts of the apparatus earthed. 
The charge on the central system and dise was now CV’, where C is the 
capacity to be determined and V’ the potential found by the position 
of the gold leaf and from the calibration curve. In the apparatus 
employed r=5-4 em., d—1:5 cm., V/V’ = 3-25, whence C= 15-8! 
At 91 cm. the y rays from 8-6 mg.Ra caused an ionization current 
of 13-5 volts/min., and as the volume of the electroscope was 6,100 
cm°., q — 3,960. In this part of the experiment, it is necessary to sur- 
