1920. ] Measurements with Quadrant Electrometers. 15 
of leakage of moist air. The upper part (A) is made of light 
brass, about 05 mm. thick, 22 cms. in diameter, and ; 
high. This is removable, and fits into a groove 1 cm. wide by 2 
cms. deep turned on the rim of a heavy cast-iron or brass base 
(C). Iron is on the whole preferable, as brass even when 
protected by paint is liable sooner or later to be attacked by 
the mercury. e groove is partly filled with mercury and 
oil, so that hermetical sealing is at once obtained with this 
arrangement when the cover is aap: in position. The 
fittings a the removable case are: 
(D) A microscope slide sealed veh Canada Balsam into a 
metal frame soldered to the case. These slides when of good 
quality are lead enough to give a sharp image when light 
passing through them is reflected from the mirror of = electro- 
meter, as in the ordinary lamp and scale arrangemen 
lantern-slide or a piece of ordinary ae glass 
about 8cms. square, similarly sealed into the case, which serves 
