The electroscope was used in conjunction with an oil lamp or gas flame.
At small heights Exner (27) has employed captive balloons, provided with a burning fuse, and carrying a wire connected with an electroscope on the ground.
For this purpose the following apparatus should be provided: - (i) two small metal tea-trays and some clean dry tumblers, the latter preferably varnished with shellac varnish made with alcohol free from water; (2) two sheets of ebonite rather larger than the tea-trays; (3) a rod of sealing-wax or ebonite and a glass tube, also some pieces of silk and flannel; (4) a few small gilt pith balls suspended by dry silk threads; (5) a gold-leaf electroscope, and, if possible, a simple form of quadrant electrometer (see Electroscope and Electrometer); (6) some brass balls mounted on the ends of ebonite penholders, and a few tin canisters.
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.
The sign of the electrification imparted to the electroscope when so charged - that is, whether positive or negative - can be determined by rubbing the sealing-wax rod with flannel and the glass rod with silk, and approaching them gently to the electroscope one at a time.
The leaves of the electroscope will diverge with positive electricity.
On the plate of a gold-leaf electroscope place a metal canister having a loose lid.
It will be found that as it does so the gold-leaves of the electroscope diverge, but collapse again if the ball is withdrawn.
They discharge a charged electroscope, the latter, however, much more feebly than the former.
ELECTROSCOPE, an instrument for detecting differences of electric potential and hence electrification.
The earliest form of scientific electroscope was the versorium or electrical needle of William Gilbert (1544- 1603), the celebrated author of the treatise De magnete (see Electricity).
Henley and others devised the pith ball, or double straw electroscope (fig.
Faraday, however, showed long subsequently that to bestow upon the indications of such an electroscope definite meaning 1 See the English translation by the Gilbert Club of Gilbert's De magnete, p. 49 (London, 1900).
The uses of an electroscope are, first, to ascertain if any body is in a state of electrification, and secondly, to indicate the sign of that charge.
In connexion with the modern study of radioactivity, the electroscope has become an instrument of great usefulness, far outrivalling the spectroscope in sensibility.
Radio-active bodies are chiefly recognized by the power they possess of rendering the air in their neighbourhood conductive; hence the electroscope detects the presence of a radio-active body by losing an electric charge given to it more quickly than it would otherwise do.
A third great use of the electroscope is therefore to detect electric conductivity either in the air or in any other body.
To detect electrification it is best to charge the electroscope by induction.
If an electrified body is held near the gold-leaf electroscope the leaves diverge with electricity of the same sign as that of the body being tested.
If, without removing the electrified body, the plate or knob of the electroscope is touched, the leaves collapse.
If the electroscope is insulated once more and the electrified body removed, the leaves again diverge with electricity of the opposite sign to that of the body being tested.
The sign of charge is then determined by holding near the electroscope a glass rod rubbed with silk or a sealingwax rod rubbed with flannel.
When employing a Volta condensing electroscope, the following is the method of procedure: - The top of the electroscope consists of a flat, smooth plate of lacquered brass on which another plate of brass rests, separated from it by three minute fragments of glass or shellac, or a film of shellac varnish.
Volta made use of such an electroscope in his celebrated experiments (1790-1800) to prove that metals placed in contact with one another are brought to different potentials, in other words to prove the existence of so-called contact electricity.
To employ the electroscope as a means of detecting radio-activity, we have first to test the leakage quality of the electroscope itself.
Formerly it was usual to insulate the rod of the electroscope by passing it through a hole in a cork or mass of sulphur fixed in the top of the glass vessel within which the gold leaves were suspended.
This insulation, however, is not sufficiently good for an electroscope intended for the detection of radio-activity; for this purpose .:..................... ?,:.?
And Mme P. Curie employed an electroscope made as follows: A metal case (fig.
If a charge is given to the electroscope, and if any FIG.
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.
A very similar form of electroscope was employed by J.
The electroscope is provided with a charging rod C. In a dry atmosphere sulphur or amber is an early perfect insulator, and hence if the air in the interior of the box is kept dry by calcium chloride, the electroscope will hold its charge for a long time.
Another type of sensitive electroscope is one devised by C. T.
In the use of all these electroscopic instruments it is essential to bear in mind (as first pointed out by Lord Kelvin) that what a gold-leaf electroscope really indicates is the difference of potential between the gold-leaf and the solid walls enclosing the air space in which they move.'
As already mentioned, Faraday remedied this defect by coating the inside of the glass vessel in which the gold-leaves were suspended to form an electroscope with tinfoil (see fig.
In the case of conductors of the first class he proved by the use of the condensing electroscope, aided probably by some form of multiplier or doubler, that a difference of potential (see Electrostatics) was created by the mere contact of two such conductors, one of them being positively electrified and the other negatively.
It was then found that when the end plates of Volta's pile were connected to an electroscope the leaves diverged either with positive or negative electricity.
Well as of discharging a charged electroscope (Com.
The gold-leaf electroscope invented by Abraham Bennet (see Electroscope) can in like manner, by the addition of a scale to observe the divergence of the gold-leaves, be made a repulsion electrometer.