298 ' Dr. J. Hopkinson on the 
affects the idiostatic gauge in the same way. Let the jar be 
charged till the gauge comes to the mark. Call P the differ- 
ence of potential between the aluminium lever of the idiostatic 
gauge and the brass disk below which attracts it. The dit- 
ference of potential between the brass of the case and the brass 
work of the interior is P+, and between the case and the alu- 
minium needle within the quadrantsit is P+2«. If, however, 
the charge is negative, the difference is —P+2z. Hence the 
sensibility will be different from two causes, according as the 
jar is charged positively or negatively, till the idiostatic gauge 
is at its standard. For determining the constant \ we must 
take the mean of the two results 2535 and 247, that is 250°25. 
Comparing this with the actual standard charge of the jar, and 
the double deflection given by one volt 172°4, when charged 
to the standard, we see that the irregularity has not been 
wholly eliminated. It appeared desirable to determine the — 
sensibility of the instrument for a lower known charge. The 
charge was determined exactly as described above, and was 
found to be 609 volts; whilst 1 volt gave 107:1 scale-divisions 
double deflection ; whence in the equation 
p—A—B)C 
eee 
we have, if | 
Ne OMsla G7 
the following as the calculated and observed deflections :-— 
Calculated 27.0.5 e200 705. NO (ete legos 
Observed #9. 4. 2002) | LO alge 
which is well within errors of observation. 
This deviation from proportionality of sensibility did not 
appear to be worth correcting, as | was not sure that other 
small irregularities might not be introduced by raising the 
needle above the middle position within the quadrants. It 
appears probable that the small deviation still remaining does 
not arise from the attraction of the quadrants on the needle 
increasing the tension of the suspension, but from some cause 
of a quite different nature, for if it were so caused the capacity- 
equations would be 
O,=quA—q12B — 9336 —gu.D + «dC, &c., 
where 
» 
o> ae 
(A—B) (c-*+*) 
Now the experiments I have tried for determining 911, qi, 
