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ELECTROSCOPE

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Originally appearing in Volume V09, Page 240 of the 1911 Encyclopedia Britannica.
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ELECTROSCOPE , an See also:

instrument for detecting See also:differences of electric potential and hence electrification. The earliest See also:form of scientific electroscope was the versorium or See also:electrical See also:needle of See also:William See also:Gilbert (1544– 1603), the celebrated author of the See also:treatise De magnete (see See also:ELECTRICITY). It consisted simply of a See also:light metallic needle balanced on a See also:pivot like a See also:compass needle. Gilbert em- ployed it to prove that numerous other bodies besides See also:amber are susceptible of being electrified by See also:friction.' In this See also:case the visible indication consisted in the attraction exerted between the electrified See also:body and the light pivoted needle which was acted upon C and electrified by See also:induction. The next See also:im- provement was the invention of See also:simple forms of repulsion electroscope. Two similarly electrified bodies repel each other. See also:Benjamin See also:Franklin employed the repulsion of two See also:linen threads, C. F. de C. du See also:Fay, J. See also:Canton, W. See also:Henley and others devised the See also:pith See also:ball, or See also:double See also:straw electroscope (fig. 1). T.

See also:

Cavallo about 1770 employed two See also:fine See also:silver wires terminating in pith balls suspended in a See also:glass See also:vessel having strips of See also:tin-See also:foil pasted down the sides (fig. 2). The See also:object of the See also:thimble- shaped See also:dome was to keep moisture from the the pith balls were supported, so that the apparatus could be used in the open See also:air even in the See also:rainy See also:weather. See also:Abraham Bennet (Phil. Trans., 1787, 77, p. 26) invented the See also:modern form of See also:gold-See also:leaf electroscope. Inside a glass shade he fixed to an insulated See also:wire a pair of strips of gold-leaf (fig. 3). The wire terminated in a See also:plate or knob outside the vessel. When an electrified body was held near or in contact with the knob, repulsion of the gold leaves ensued. See also:Volta added the See also:condenser (Phil. Trans., 1782), which greatly increased the See also:power of the instrument.

M. See also:

Faraday, however, showed See also:long subsequently that to bestow upon the indications of such an electroscope definite meaning 1 See the See also:English See also:translation by the Gilbert See also:Club of Gilbert's De magnete, p. 49 (See also:London, 1900).it was necessary to See also:place -a See also:cylinder of metallic See also:gauze connected to the See also:earth inside the vessel, or better still, to See also:line the glass shade with tin-foil connected to the earth and observe through a hole the indications of the gold leaves (fig. 4). Leaves of See also:aluminium foil may with See also:advantage be substituted for gold-leaf, and a See also:scale is sometimes added to indicate the angular divergence of the leaves. The uses of an electroscope are, first, to ascertain if any body is in a See also:state of electrification, and secondly, to indicate the sign of that See also:charge. In connexion with the modern study of radio-activity, the electroscope has become an instrument of See also: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 deter-See also:

mined by holding near the electroscope a glass See also:rod rubbed with See also:silk or a sealing-See also:wax rod rubbed with See also:flannel. If the approach of the glass rod causes the leaves in their final state to collapse, then the charge in the rod was See also:positive, but if it causes them to expand still more the charge was negative, and See also:vice versa for the sealing-wax rod. When employing a Volta condensing electroscope, the following is the method of See also:procedure:—The See also:top of the electro- FIG. 4.–Gold-Leaf See also:scope consists of a See also:flat, smooth plate Electroscope. of lacquered See also:brass on which another plate of brass rests, separated from it by three See also:minute fragments of glass or shellac, or a film of shellac See also:varnish. If the electrified body is touched against the upper plate whilst at the same See also:time the See also:lower plate is put to earth, the condenser formed of the two plates and the film of air or varnish becomes charged with positive electricity on the one plate and negative on the other. On insulating the lower plate and raising the upper plate by the glass handle, the capacity of the condenser formed by the plates is vastly decreased, but since the charge on the lower plate including the gold leaves attached to it remains the same, as the capacity of the See also:system is reduced the potential is raised and therefore the gold leaves diverge widely. 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. He was assisted to detect the small potential differences then in question by the use of a multiplying condenser or revolving doubler (see ELECTRICAL See also:MACHINE). 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 See also:cork or See also:mass of See also:sulphur fixed in the top of the glass vessel within which the gold leaves were suspended.

A further improvement consisted in passing the See also:

metal wire to which the gold leaves were attached through a glass See also:tube much wider than the rod, the latter being fixed concentrically in the glass tube by means of solid shellac melted and run in. This insulation, however, is not sufficiently See also:good for an electroscope intended for the detection of radio-activity; for this purpose Electroscope. See also:stem from which it must be such that the leaves will remain for See also:hours or days in a state of steady divergence when an electrical charge has been given to them. In their researches on radio-activity M. and Mme P. See also:Curie employed an electroscope made as follows:—A metal case (fig. 5), having two holes in its sides, has a See also:vertical brass See also:strip B attached to the inside of the lid by a See also:block of sulphur SS or any other good insulator. Joined to the strip is a transverse wire terminating at one end in a knob C, and at the other end in a condenser plate P'. The strip B carries also a strip of gold-leaf L, and the metal .case is connected to earth. If a charge is given to the electroscope, and if any radio-active material is placed on a troscope. condenser plate P attached to the See also:outer case, then this substance be-stows conductivity on the air between the plates P and P', and the charge of the electroscope begins to leak away. The collapse of the gold-leaf is observed through an See also:aperture in the case by a miscroscope, and the time taken by the gold-leaf to fall over a certain distance is proportional to the ionizing current, that is, to the intensity of the radio-activity of the substance. A very similar form of electroscope was employed by J. P.

L. J. See also:

Elster and H. F. K. Geitel (fig. 6), and also by C. T. R. See also:Wilson (see Proc. See also:Roy. See also:Soc., 1901, 68, p.

152). A metal See also:

box has a metal strip B suspended from a block or insulator by means of a See also:bit of sulphur or amber S, and to it is fastened a strip of gold-leaf L. The electroscope is provided with a charging rod C. In a dry See also:atmosphere sulphur or amber is an See also:early perfect insulator, and hence if the air in the interior of the box is kept dry by See also:calcium chloride, the electroscope will hold its charge for a long time. Any divergence or collapse of the gold-leaf can be viewed by a See also:microscope through an aperture in the See also:side of the case. Another type of sensitive electroscope is one devised by C. T. R. Wilson (Prot. See also:Cam. Phil. Soc., 1903, 12, See also:part 2).

It consists of a metal box placed on a tilting stand (fig. 7). At one end is an insulated plate P kept at a potential of 200 volts or so above the earth by a See also:

battery. At the other end is an insulated metal wire having attached to it a thin strip of gold-leaf L. If the plate P is electrified it attracts the strip which stretches out towards it. Before use the strip is for one moment connected to the case, and the arrangement is then tilted until the strip extends at a certain See also:angle. If then the strip of gold-leaf is raised or lowered in potential it moves to or from the plate P, and its See also:movement can be observed by a microscope through a hole in the side of the box. There is a particular angle of tilt of the case which gives a maximum sensitiveness. Wilson found that with the plate electrified to 207 volts and with a tilt of the case of 300, if the gold-leaf was raised one volt in potential above the case, it moved over 200 divisions of the See also:micrometer scale in the See also:eye-piece of the micro-scope, 54 divisions being equal to one millimetre. In using the instrument the insulated rod to which the gold-leaf is attached is connected to the conductor, the potential of which is being examined. In the use of all these electroscopic See also:instruments it is essential to See also:bear in mind (as first pointed out by See also:Lord See also: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.' If these enclosing walls are made of anything else than perfectly conducting material, then the indications of the instrument may be uncertain and meaning-less. 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.

4). In spite of these admonitions all but a few instrument makers have continued to make the vicious type of instrument consisting of a pair of gold-leaves suspended within a glass shade or See also:

bottle, no means being provided for keeping the walls of the vessel continually at zero potential. See J. Clerk See also:Maxwell, Treatise on Electricity and See also:Magnetism, vol. i. p. 300 (2nd ed., See also:Oxford, 1881); H. M. Noad, A See also:Manual of Electricity, vol. i. p. 25 (London, 1855); E. See also:Rutherford, Radio-activity. (J. A.

End of Article: ELECTROSCOPE

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