11] METHODS OF MEASUREMENT 81 
In course of time the walls of the room become radio-active owing 
to the dissemination of dust and the action of the radio-active 
emanations. 
58. Electrometer key. For work with electrometers of 
high sensibility, a special key is 
necessary to make and break from 
a distance the connection of the 
quadrants to earth in order to 
avoid electrostatic disturbances at 
the moment the current is to be 
measured. The simple key shown 
in Fig. 15 has been found very 
satisfactory for this purpose. <A 
small brass rod BM, to which a 
string is attached, can be moved 
vertically up and down in a brass —-Eletrometer Testing Vessel 
tube A, which is rigidly attached Fig. 15. 
to a bent metal support connected 
to earth. When the string is released this rod makes contact with 
the mercury M, which fills a hole in the small block of ebonite P. 
The electrometer and testing vessel are connected with the mercury. 
When the string is pulled the rod BM is removed from the 
mercury and the earth connection of the electrometer system 1s 
broken. On release of the string, the rod BM falls and the electro- 
meter is again earthed. By means of this key, which may be 
operated at any distance from the electrometer, the earth con- 
nection may be made and broken at definite intervals without 
any appreciable disturbance of the needle. 

59. Testing apparatus. The arrangement shown in Fig. 16 
iS very convenient for many measurements in radio-activity. Two 
parallel insulated metal plates A and B are placed inside a metal 
vessel V, provided with a side door. The plate A is connected with 
one terminal of a battery of small storage cells, the other pole of 
which is earthed; the plate 6 with the electrometer, and the vessel 
V with earth. The shaded areas in the figure indicate the position 
of ebonite insulators. The active material to be tested is spread 
uniformly in a shallow groove (about 5 cms. square and 2 mms. 
R. B.-A. 6 
