an instrument for detecting very small charges of electricity, electric fields, or radiation: it can indicate whether they are positive or negative, as by the divergence of electrically charged strips of gold leaf: when fitted with optical means for quantitative observation of the divergence, an electroscope serves as an electrometer
The electroscope was used in conjunction with an oil lamp or gas flame.
Condenser plate P attached to the outer case, then this substance bestows conductivity on the air between the plates P and P', and the charge of the electroscope begins to leak away.
If an insulated brass ball is touched against the first tray and then against the knob or plate of the electroscope, the gold leaves will diverge.
If the ball is discharged and touched against the other tray, and then afterwards against the previously charged electroscope, the leaves will collapse.
If whilst holding the ebonite sheet over the tray the latter is also touched with an insulated brass ball, then this ball when removed and tested with the electroscope will be found to be negatively electrified.