Electroscopes

11.] An electroscope is an instrument by means of which the existence of electrification may be detected. All electroscopes are capable of indicating with more or less accuracy not only the existence of electrification, but its amount.

Such indications, however, though sometimes very useful in guiding the experimenter, are not to be regarded as furnishing a numerical measurement of the electrification. Instruments so constructed that their indications afford data for the numerical measurement of electrical quantities are called Electrometers.

An electrometer may of course be used as an electroscope if it is sufficiently sensitive to indicate the electrification in question, and an instrument intended for an electroscope may, if its indications are sufficiently uniform and regular, be used as an electrometer.

The class of electroscopes of simplest construction is that in which the in- dicating part of the instrument consists of two light bodies suspended side by side, which, when electrified, repel each other, and indicate their electrifica- tion by separating from each other.

The suspended bodies may be balls of elder pith, gilt, and hung up by fine linen threads (which are better conductors than silk or cotton), or pieces of straw or strips of metal, and in the latter case the metal may be tinfoil or gold-leaf, thicker or thinner according to the amount of electrification to be measured.

We shall suppose that our electroscope is of the most delicate kind, in which gold leaves are em- ployed (see Fig. 1). The indicating apparatus l, l, is generally fastened to one end of a rod of metal L, which passes through an opening in the top of a glass vessel G. It then hangs within the vessel, and is protected from currents of air which might pro- duce a motion of the suspended bodies liable to be mistaken for that due to electrification. To test the electrification of a body the electri- fied body is brought near the disk L at the top of the metal rod, when, if the electrification is strong enough, the suspended bodies diverge from one an- other.

Fig. 1.

The glass case, however, is liable, as Faraday pointed out, to become itself electrified, and when glass is electrified it is very difficult to ascertain by experiment the amount and the distribution of its electrification.

There is thus introduced into the experiment a new force, the nature and amount of which are unknown, and this interferes with the other forces acting on the gold leaves, so that their divergence can no longer be taken as a true indication of their electrification. The best method of getting rid of this uncertainty is to place within the glass case a metal vessel which almost surrounds the gold leaves, this vessel being connected with the earth.

When the gold leaves are electrified the in- side of this vessel, it is true, becomes oppositely electrified, and so increases the divergence of the gold leaves, but the distribution of this electrification is always strictly dependent on that of the gold leaves, so that the divergence of the gold leaves is still a true indication of their actual electrical state. A continuous metal vessel, however, is opaque, so that the gold leaves cannot be seen from the outside. A wire cage, however, may be used, and this is found quite sufficient to shield the gold leaves from the action of the glass, while it does not prevent them from being seen.

The disk, L, and the upper part of the rod which supports the gold leaves, and another piece of metal M, which is connected with the cage m, m, and extends beyond the case of the instrument, are called the electrodes, a name invented by Faraday to denote the ways by which the electricity gets to the vital parts of the instrument.

The divergence of the gold leaves indicates that the potential of the gold leaves and its electrode is different from that of the surrounding metal cage and its electrode. If the two electrodes are connected by a wire the whole instrument may be electrified to any extent, but the leaves will not diverge. Experiment V.

The divergence of the gold leaves does not of itself indicate whether their potential is higher or lower than that of the cage; it only shews that these potentials are different. To ascertain which has the higher potential take a rubbed stick of sealing-wax, or any other substance which we know to be negatively electrified, and bring it near the electrode which carries the gold leaves.

If the gold leaves are negatively electrified they will diverge more as the sealing-wax approaches the rod which carries them; but if they are positively electrified they will tend to collapse. If the electrification of the sealing-wax is considerable with respect to that of the gold leaves they will first collapse entirely, but will again open out as the sealing-wax is brought nearer, shewing that they are now negatively electrified.

If the electrode M belonging to the cage is insulated from the earth, and if the sealing-wax is brought near it, the indications will be exactly reversed; the leaves, if electrified positively, will diverge more, and if electrified negatively, will tend to collapse.

If the testing body used in this experiment is positively electrified, as when a glass tube rubbed with amalgam is employed, the indications are all re- versed.

By these methods it is easy to determine whether the gold leaves are pos- itively or negatively electrified, or, in other words, whether their potential is higher or lower than that of the cage.

12.] If the electrification of the gold leaves is considerable the electric force which acts on them becomes much greater than their weight, and they stretch themselves out towards the cage as far as they can. In this state an increase of electrification produces no visible effect on the electroscope, for the gold leaves cannot diverge more. If the electrification is still further increased it often happens that the gold leaves are torn off from their support, and the instrument is rendered useless∗.

It is better when we have to deal with high degrees of electrification to use a less delicate instrument. A pair of pith balls suspended by linen threads answers very well; the threads answer sufficiently well as conductors of electricity, and the balls are repelled from each other when electrified.

For very small differences of potential, electroscopes much more sensitive than the ordinary gold-leaf electroscope may be used.

Thomson’s Quadrant Electrometer.

13.] In Sir William Thomson’s Quadrant Electrometer the indicating part consists of a thin flat strip of aluminium (see Fig. 2) called the needle, at- tached to a vertical axle of stout platinum wire. It is hung up by two silk fibres x, y, so as to be capable of turning about a vertical axis under the ac- tion of the electric force, while it always tends to return to a definite position of equilibrium. The axis carries a concave mirror t by which the image of a flame, and of a vertical wire bisecting the flame, is thrown upon a graduated

[For the sake of safety the cage is often so arranged that the gold leaves touch it and become discharged before diverging to their extreme limit.]QUADRANT ELECTROMETER.

scale, so as to indicate the motion of the needle round a vertical axis. The lower end of the axle dips into sulphuric acid contained in the lower part of the glass case of the instrument, and thus puts the needle into electrical connection with the acid. The lower end of the axle also carries a piece of platinum, immersed in the acid which serves to check the vibrations of the needle. The needle hangs within a shallow cylindrical brass box, with circu- lar apertures in the centre of its top and bottom. This box is divided into four quadrants, a, b, c, d, which are separately mounted on glass stems, and thus insulated from the case and from one another. The quadrant b is removed in the figure to shew the needle. The position of the needle, when in equilib- rium, is such, that one end is half in the quadrant a and half in c, while the other end is half in b and half in d. The electrode l is connected with the quadrant a and also with d through the wire w. The other electrode, m, is connected with the quadrants b and c. The needle, u, is kept always at a high po- tential, generally positive. To test the differ- ence of potential between any body and the earth, one of the electrodes, say m, is con- nected to the earth, and the other, l, to the body to be tested. The quadrants b and c are therefore at po- tential zero, the quadrants a and d are at the potential to be tested, and the needle u is at a high positive potential. The whole surface of the needle is electri- fied positively, and the whole inner surface of the quadrants is electrified negatively, but the greatest electrification and the greatest attraction is, other things being equal, where the difference of potentials is greatest. If, therefore, the potential of the quadrants a and d is positive, the needle will move from a and d towards b and c or in the direction of the hands of a watch. If the potential of a and d is negative, the needle will move towards these quadrants, or in the opposite direction to that of the hands of a watch. The higher the potential of the needle, the greater will be the force tend- ing to turn the needle, and the more distinct will be the indications of the instrument.

Fig. 2.