296 Dr. J. Hopkinson on the 
which I shall now describe cannot be directly compared with 
what goes before. The old experiment was first repeated, and 
the existence of a maximum sensibility again found. On 
examination, it was found that the needle hung a little low so 
that it was nearer to the part of the quadrant below it than to 
that above. It is easy to see that this would produce the 
anomalous result observed, though there is reason for thinking 
it is not the sole cause. The effect of the needle being low 
is that it will be on the whole attracted downwards; and so the 
apparent weight hanging on the fibre-suspension and the con- 
sequent tension of the fibres will be increased. The increase 
of the tension will be as the square of the potential C; and 
hence the formula for the deflection will be modified to 
r A+B 
io; (c- x) 
where & is a constant depending upon the extent to which the 
position of the needle deviates from its true position of 
midway between the upper and lower parts of the quadrants. 
By a proper choice of &, the results I previously obtained are 
found to agree well with this formula. 
The electrometer was next adjusted in the following way:— 
The needle was raised by taking up the fibres of the suspen- 
sion and adjusting them to equal tension in the usual way, 
and the proportionality of sensibility to charge was tested, the 
charge being now determined in arbitrary units by discharging 
the jar of the instrument through a ballistic galvanometer. 
The operation was repeated until the sensibility, so far as this 
method of testing goes, was proportional to the charge of the 
jar over a very long range. It was then found that the 
needle was slightly above the median position within the 
quadrants. Increased tension of the fibres from electrical 
attraction does not therefore account for the whole of the 
facts, although it does play the principal part. The sensibility 
of the instrument being now at least approximately propor- 
tional to the charge of the jar, I proceeded to determine 
accurately the potential of the jar when charged to the standard 
as indicated by the idiostatic gauge. 
In what follows the quadrants, one of which is under the 
induction-plate, are denoted by B, the others by A. The 
quadrants B are connected to the case, A are insulated. The 
jar is connected to the induction-plate, and the reading on the 
scale noted ; the connection is broken, and the induction- 
plate is connected to the case, and the reading on the scale 
again noted; the difference is the deflection due to the charge 
in the jar. It is necessary to read the scale for zero-charge 
j= 
