order that 20, 30, and 50 amperes through the strip may produce 100 divisions' deflection.

54. Gold-Leaf Electroscope.-If we desire to measure the P.D. between two insulated bodies which have been electrified by touching them, for example, one with a rubbed piece of ebonite, and the other with a rubbed piece of glass, it would be impossible to employ any form of current voltmeter. For no matter how fine or how long were the wire used in winding the galvanometer, or how large was the resistance of the added wire w (Fig. 86), the flow of electricity which enabled the P.D. to be indicated would at once destroy the very P.D. we desired to measure. An electrostatic voltmeter must, therefore, be employed in such a case, but as there is no difficulty in producing a P.D. of many hundreds of volts by means of rubbed ebonite or rubbed glass, the voltmeter may, for many purposes, be of a much rougher kind than the one already described.

When it is only required to know whether one potential is higher, or lower, than another, or whether the potential of a body is plus or minus, that is to say, whether a positive current would flow from the body to the ground, or from the ground to the body, if they were connected together by a wire, such a qualitative test can be conveniently made with a "gold-leaf electroscove."

This instrument, as formerly constructed, had a

variety of faults, but the illustrated description that was given, in the earlier editions of this book, of the proper way to construct a gold-leaf electroscope, has induced some manufacturers, at any rate, to cease reproducing instruments possessing the glaring defects of the older types. In the present edition of the book it will be,

therefore, sufficient to describe the way in which a goldleaf electroscope may be satisfactorily constructed.

A glass shade GG (Fig. 93) rests on a wooden base, and is covered inside with the conducting varnish devised by Mr. Mather and the author (see § 58, page 199), or with strips of tin-foil T, placed only just so far apart as is necessary to enable the gold leaves to be easily seen. These strips of tin-foil are bent round the bottom of the

glass shade, and connected electrically with a brass ring, which encircles the outside of the bottom of the glass shade. To this ring three horizontal brass lugs are attached for enabling the shade to be screwed to the wooden base, and to one of them is fixed a binding screw, s, for holding any wire which we wish to electrically connect with the tin-foil coating. Inside the glass shade G G, thin rods of good insulating glass g, g are cemented into two short brass tubes, or collars, fixed to the base, and the glass rods are joined together at the top by being cemented into a little metallic tube tt, carrying the thick wire w w, and the gold leaves L. This wire w passes through the top of the instrument without touching it, and may carry at its top a little knob or a little binding-screw. v is a glass vessel containing lumps of pumice-stone soaked in strong sulphuric acid, which absorbs any water vapour in the interior of the electrometer, and thus keeps the glass rods g, g dry.

When the instrument is not in use the little metal plug or stopper p (which is made to slide a little stiffly on the wire w by the hole in the stopper being lined with cork) should always be pushed down, and the hole at the top of the instrument thus closed to keep out dust and damp. If this precaution be carefully attended to on every occasion that the electroscope is left unused even for a short time, and the surface of the glass rods g, g be initially carefully cleaned, the insulation of the instrument will remain so good, even for a year after the acid has been put on to the pumice-stone, that an electric charge given at any time to the gold leaves will remain practically undiminished by leakage during an hour even on a very damp day.

With a given gold-leaf electroscope the divergence of the gold leaves depends simply on the P.D. between the gold leaves L and the tin foil coating T. For the gold leaves constitute a flexible needle corresponding with N in Fig. 83, page 164, and the tin-foil coating is the stationary inductor (called I in the same figure) to which the gold leaves are attracted with a force depending on the P.D.

H

between them and the tin-foil coating. This attraction causes the leaves to diverge, and to be, therefore, lifted ; the angle of divergence for any particular P.D. being such that the attractive forces exactly balance the controlling forces introduced by the weight of the leaves which are slightly lifted from the vertical position. A gold-leaf electroscope is, therefore, a "deflectional gravity-voltmeter.”

55. Sensibility of Gold-Leaf Electroscopes.-As already explained, gold-leaf electroscopes are frequently used merely as qualitative instruments, but employing method No. 2, § 52, page 184, a gold-leaf electroscope may be calibrated, if desired, by comparison with the zero electrostatic voltmeter (Fig. 83). The law connecting the divergence of the leaves with the P.D. set up between them and the case depends on three things (1) the length of the leaves, (2) the weight per square inch of the leaf, and (3) the size of the case. If the length of the leaves and the size of the case be fixed, it follows, from our original definition of what is meant by one P.D. being twice another, that the P.D. required to produce any particular divergence is simply proportional to the square root of the weight of the leaf per square inch.

Specimens of gold leaf from different gold-beaters appear to vary as much as 20 per cent. in the weight per square inch, but the lighter the leaf the lower will be the price, provided that it is not much below 40 shillings per book of 1,000 leaves, in which case cheapness may result from the impurity and not from the thinness of the gold. At 40 shillings per thousand sheets of 22 carat gold, the sheets being 34 inches square, the weight per square inch is about 0·013 grain. With leaves, each

21 inches long, cut from this quality of material and suspended in a conducting case 4 inches internal diameter, a divergence of about 56° is obtained for a P.D. of 1,000 volts, set up between the leaves and the case. Reducing the length of the leaves to 1 inch increases the divergence for the same P.D. to 60° and in addition it renders the various divergences between the leaves

in degrees more nearly directly proportional to the P.D. in volts.

The calibration curve can also be rendered much more nearly a straight line by increasing the diameter of the case, but this has the counterbalancing effect of diminishing the sensibility for the same leaves, as may be seen from the following table :

LEAVES EACH 1 INCH LONG. P.D. OF 1,000 VOLTS MAINTAINED BETWEEN LEAVES AND CASE.

Plotting a curve to represent the above four pairs of values and continuing the curve forwards, it is seen that the divergence rapidly approaches 40°, which means that however large may be the diameter of the conducting case the divergence will be about 40° when a P.D. of 1,000 volts is maintained between this case and a pair of leaves each 1 inch long cut from a 40-shilling book of 22 carat gold leaf.

With the leaves each 1 inch long the case can be made as narrow as 42 inches in diameter and still nearly direct proportionality of P.D. and divergence be obtained up to 70° whatever be the weight of the leaves. This is the size of leaf and case, therefore, that may be conveniently adopted, and the constant of instruments so constructed will vary from about 6° per 100 volts to 6° per 225 volts, as the material used in making the leaves costs 40 shillings per 1,000 sheets, or a few pence when the material is "Dutch metal."