400 
PHYSICS: A. L. PARSON 
interfering with difference of path of over a million waves, such a grating 
would have a resolving power exceeding a million. 
This investigation has had assistance from the Bache Fund of the 
National Academy of Science, from the Carnegie Institution, and from 
the University of Chicago. In addition to the grateful acknowledge- 
ment to these institutions I would add my high appreciation of the 
faithful services rendered by Messrs. JuKus Pearson and Fred Pearson. 
A HIGHLY SENSITIVE ELECTROMETER 
By A. L. Parson 
CHEMICAL LABORATORY. UNIVERSITY OF CALIFORNIA 
Preiented to the Academy. June 3, 1915 
In this instrument the principle of working in a condition approaching 
instabihty is used to increase greatly the sensitiveness of the quadrant 
electrometer, which, even in its ordinary form, is more sensitive than 
other electrometers in which this condition of instability has been em- 
ployed. The box-shaped quadrants are replaced by flat sectors subtend- 
ing about 70° each at the centre and arranged 
as shown in the figure. Because of the two 
large gaps between A and B', and between A' 
and B, the needle hangs stably, as shown by 
the dotted lines, by virtue of electric forces 
quite apart from the torsion oi the suspen- 
sion. (Incidentally, this makes it possible to 
use a torsionless suspension or a float for the 
needle, which latter has been tried with some 
success.) If now a potential-difference is set 
up between AA'and BB' the needle is deflected, 
at first nearly proportionately to the potential 
difference, and then less and less as the large gaps are approached. 
The approach to instabihty and hence an increase in sensitiveness is 
brought about by means of a mechanical device, which on turning a 
micrometer screw can be made to widen the small gaps between A and 
B and between A' and B', thus making the needle less and less stable 
in its central symmetrical position. In this way the sensitivity which 
in the least sensitive condition is about 3 mm. per milKvolt at a scale 
distance of 5 metres can be raised to as much as 60 mm. per miUivolt 
easily, and with some care even to 150 mm. per millivolt. 
A most important feature of the instrument is that, on account of the 
relative unimportance of the thickness of the quartz suspension, the 
A mOHLY SENSITIVE 
ELECTROMETER 
