Discharg

consult the account given by Mascart, Trailé d' Electricité Statique, tom. i. §§ 313-316, and Riess, l.c.

The torsion balance of Coulomb is another instrument suited for the direct measurement of electrical quantity. For its construction and use see the article ELECTRICITY, p. 18.

The discharging electroscope of Gaugain belongs to the present ing e' c class of instruments. It consists (fig. 2) of an ordinary (oldtroso pe. fashioned) gold-leaf electroscope, with the addi tion of a small knob B, connected with the metal solo of the instrument, and standing a little to one side of one of the leaves. The charge on any conductor is measured by connecting it with the knob A through a sufficient length of wet cotton to retard the discharge properly. When a certain amount of electricity has reached the gold leaf, it is attracted to the knob B and is discharged; it then falls back, is recharged, then discharged by contact with B a second time, and so on. It is found that the same quantity of electricity is discharged at each contact if the process be properly regulated; so that the whole Fia. 2. -Discharg charge on the conductor is measured by the ing Electroscope number of oscillations of the gold leaf required

electro

to discharge it completely.'

BW

The rest of the instruments (save one) to be described may be classified under the three heads given by Sir Wm. Thomson in his valuable report on electrometers,2 viz., (1) repulsion electrometers, (2) attracted disc electrometers, and (3) symmetrical electrometers.

1. Repulsion Electrometers.-The electroscopic needle of Gilbert is the oldest specimen of a repulsion electroscope. The linen threads of Franklin, and the double pendulum used by Canton, Du Fay, and others, which was an improvement thereon, are typical of another species of electroscope coming under the same genus.

Cavallo's electroscope (fig. 3) embodies the double pendulum principle. It consists of Cavallo's two fine silver wires loaded with small pieces of cork scope. or pith, and suspended inside a small glass cylinder. Through the cap which closes the cylinder passes the stout wire from which the pendulums are Buspended. This wire ends in a thimble-shaped dome A, which comes down very nearly to the cap; the outside of the cap and part of the wire aro covered with sealing wax, and the object of the dome is to keep moisture from the stem, so that the electroscope could be used in the open air even in rainy weather. To add to the sensitivo.

old-leaf lectrocope.

A

Fia. 3.-Cavallo's Electroscope.

ness of the instrument two strips of tinfoil are pasted on the glass at B and C opposite the pith balls. An electroscope similar to this was used by Saussure. Volta used a pair of straws instead of the pith ball pendulums.

By far the most perfect form of electroscope on the double pendulum principle is the gold-leaf electroscope of Bennet. Fig. 4 represents a modern form of this instrument. The gold leaves are gummed on the two sides of a flat piece of metal carried by a stout stem, which passes through the top of a glass shade and ends Jennet's in a flat disc. By means of this diso we may convert the instru ment into Volta's condensing electroscope (already described, see ELECTRICITY, P. 34). Inside the glass shade, and rising well over the leaves, stands a cylinder of wire gauze, which ought to be in metallic connection with the earth, or with some conductor whose potential is taken as the standard of reference. The introduction of the wire cylinder is due to Faraday, and is an essential improvement; it is absolutely necessary, in fact, to convert the instrument into a trustworthy indicator of differences of potential. It serves the double purpose of protecting the leaves from external disturbing influences, and of ensuring that the instrument always indicates the difference between the potential of the body connected with the

the graduation would of course have to be determined by experi ment. Peclet did, as a matter of fact, use the gold-leaf electroscope in this way.

meter

The electrometer of Henley,' sometimes called Henley's quadrant Henle electrometer (fig. 5), may be taken as the type of single pendulum electr electroscopes. It consists essentially of a pendulum A hinged to a vertical support C, which carries a vertical graduated semicircle B, by means of which the deviation of A from the vertical can be read off. This form of electroscope is, or was, much used for indicating the state of electrification of the prime conductors of electric machines. The stem is screwed into the conductor, and the divergence of the pendulum indicates roughly the charge. The sine electrometer of August, represented in fig. 6, is a modification of the single pendulum electroscope, analogous in principle Pends to Pouillet's sine compass. A is a pendulum suspended by two threads to secure motion in one plane; B is a ball fixed to the case, and connected with a suitable electrode. Any charge is given to A; B is charged with g units of electricity; the case is turned through an angle o in a vertical plane until the distance between A and B is the same as it was when both were neutral; then, if the charge on A be always the same,

q∞ sin.

This instrument is interesting on account of the principle employed in its construction; but we are not aware that it has ever been used in practice.

Another class of instruments, in FIG. 6.-Sine Electrometer. which the movable part is a horizontal arm turning about a vertical axis, may be looked upon as the descendants of Gilbert's electroscopic needle. The electrometer of Peltier and its modification into a sine electrometer (by Riess) are instruments of this class. Descriptions of both will be found in Mascart, §§ 291 and 292.

ium si

electr

meter

Dellmann's electrometer (fig. 7) is constructed on a principle Dell similar to that applied in the two instruments last named. D is a mann needle, formed of light silver wire, suspended by a fine glass fibre from a torsion head A. Below the needle is a piece of sheet metal NE, divided half through by a notch in the middle, and then bent in opposite directions on both sides of the notch, so that, when looked at end on, it appears like a Y. Underneath NE is a

It was by no means safe to take this for certain in the old instan ments, owing to the electrification of the glass. Phil. Trans., 1772.

graduated disc PL, through the centre of which passes a glass tube F supporting NE, so that it can be raised or depressed by a lever G. Inside F is a spring by means of which the lever H, which serves as electrode, can be connected or disconnected at will with the metal piece NE. The whole contained in a metal case B, the lid of which is of glass, so that the position of the reedle D on the graduation PL can be read off by means of the lens M. To use the instrument, the case is connected with the earth, the needle is brought nearly at right angles to NE, and NE is raised by

means of G till the needle is in contact with it; then the electrode K is brought into communication with NE, and the body whose charge or potential is to be measured is connected with K. The connection with K is then suppressed, and NE lowered; and the needle, now free, is repelled by NE. If, by means of the torsfon head, we bring the needle along to a fixed position relative to NE, the electrical couple will be proportional to the square of the charge communicated to NE and D, ie., to the square of the potential of the body connected with K, provided the capacity of the electrometer be negligible compared with that of the body. Hence the potential is measured by the square root of the torsion on the fibre when the needle is in a given position.

The form of Dellmann's electrometer we have just described was that used by Kohlrausch. It has been simplified by its inventor, and applied in his important investigations on atmospheric electricity.

Coulomb's balance might be used as an electrometer on the repulsion principle. Special care would, however, be necessary to avoid or to allow for disturbances arising from the case of the instrument, which ought under any circumstances to be coated wholly or par. tially with tinfoil on the inside, according to Faraday's plan. Sir Wm. Thomson did, in fact, design an electrometer of this description, and has given tables (Reprint of Papers, § 142) for reducing its indications. This type of electrometer has not come into general use. IL Allracted Disc Electrometers.-The first idea of this kind of

it will be understood from fig. 8. C is an insulated disc, over which is suspended another disc, hung from the arm of a balance, and connected with the earth. A weight w is put in a scale attached to the other arm of the balance. The insulated disc is connected with the internal armature B of a Leyn jar, whose outer armature is in connection with the suspended disc. Electricity is conveyed to B, and the quantity q measured by a small Lane's jar A, until the electric attraction at C is just sufficient to turn the balance. Snow Harris found that wx q3. This and other laws established by him agree with the mathematical theory as developed in the article ELECTRICITY.

Great improvements have been effected in this kind of electrometer by Sir Wm. Thomson-(1) by his invention of the "guard ring" or "guard plate;" (2) by using the torsion of a platinum wire for the standard force; (3) by devising proper means for attaining a definite standard potential, and by protecting the vital parts of the electrometer from extraneous disturbance; and (4) by introducing sound kinematical principles into the construction of the movable parts.

In order to illustrate these points it will be well to describe the Thom portable electrometer (fig. 9), one of his simpler instruments, in sortat detail.

portable electro

The principal electrical parts of this electrometer are sketched in meter fig 10. HH is a plane disc of metal (called the "guard plate"), kept at a constant potential by being fixed to the inner coating of a small Leyden jar M M (fig. 9), which forms the case of the instru ment. At F a square bole is cut out of HH, and into this fits, nearly as it can without danger of touching, a square piece of alumi. nium foil as light as is consistent with proper stiffness. One side of this disc is bent down, and then runs out hori.

as

zontally into a narrow stem ending in a stirrup L-the whole being not unlike a spade. The sole of the stirrup consists of a fine hair, which moves up and down before a vertical enamelled

Fia. 9-Section of Thomson's Portable
Electrometer.

piece bestridden by the
fork of the stirrup. Ou
the enamel are two small dots very near each other. When the
hair seen through a small convex lens appears straight, and bi-
sects the distance between the dots, the stirrup is said to be in
the sighted position. The alumi-
nium spade is suspended on a hori
zontal platinum wire stretched by
platinum springs at its two ends,
and is carefully balanced with its
centre of gravity in the line of
suspension, so that the only force
other than electric that can affect it
is the torsion of the wire, which
acts like the string in the toy called
the "jumping frog," or like the hair
rope in the catapulta of the ancients.
The spade is so arranged that Fis
as nearly as possible in the same
plane with the guard plate when
the hair is in the sighted position.
It is the torsional couple exerted
by the wire in this position that
Fig 10.
forms the standard force. The
remaining important electrical part is the plane horizontal disc G.
It is essential to the action of the instrument that we should be
able to move the disc G parallel to itself and to HH through
measured distances. The mechanism by which this is accomplished
is a remarkable instance of the application of geometrical principles,
to mechanism, and the reader will do well to read Thomson's
"Lesson to the instrument makers" on this subject in the Reprint
of his papers, § 369. The glass stem which carries G is fixed into
the lower end of a hollow brass cylinder; in the upper end of the
cylinder is fixed a nut AC, through which works a carefully cut
screw ending in a rounded point B of polished steel. The point B
rests on a horizontal agate plate let into a foot which projects from
See also Reprint of Sir Wm. Thomson's Papers, § 158.

2

connected with the guard plate to avoid all electrical forces on the balance; the hair of the balance is brought to the sighted position,

FIG. 13.-Dry Pile Electroscope. and the upper screw reading taken; then a weight of w grammes is distributed symmetrically on the disc, the balance brought up again by working the screw, and the reading again taken. We

thus ascertain how far the weight of w grammes depresses the balance. The weight is now removed, and the balance left at a distance above A equal to that just found. A is now charged, and its potential adjusted till the hair of the gauge indicates that the standard potential is reached. Let it now be required to measure the difference between the potentials V and V' of two conductors. Con. nect first one and then the other with n, and work the lower screw till the hair of the balance is sighted in each case, and let the screw readings reduced to centi. metres bed and d. Then, since the

Fig. 14.

force on the disc in each case is gw, where g is the acceleration produced by gravity in a falling body in centimetres per second, we have by (1)

where S denotes the area of the balance disc, or rather the mean of the areas of the disc and the hole in which it works. We thus get the value of V-V' in absolute electrostatic C. G. S. units.

III. Symmetrical Electrometers.-Two instruments fall to be Dry pile electro described under this head,-the dry pile electroscope, and Thomson's scope. quadrant electrometer. The idea common to these instruments is to measure differences of potential by means of the motions of an electrified body in a symmetrical field of force. In the dry pile electroscope, a single gold leaf is hung up in the field of force, between the opposite poles of two dry piles, or, in later forms of the instrument, of the same dry pile. The original inventor of this apparatus was Behrens, but it often bears the name of Bohnenberger, who slightly modified its form. Fechner introduced the important improvement of using only one pile, which he removed from the immediate neighbourhood of the suspended leaf. The poles of the pile are connected with two discs of metal, between which the leaf hangs. This arrangement makes it easier to secure perfect symmetry in the electric field, and allows us to vary the sensitiveness of the instrument by placing the metal plates at different distances from the leaf. In order to make the attaingat

Quadrant electrometer.

experiments in which Hankel used it are alluded to in the article ELEOTRICITY.

In the quadrant electrometer of Sir Wm. Thomson, which is the most delicate electrometric instrument hitherto invented, the moving body is a horizontal flat needle of aluminium foil, surrounded by a fixed flat cylindrical box (fig. 14), which is divided into four insulated quadrants A, B, C, D. The opposite pairs A, D and B, C are connected by thin platinum wires. The two bodies whose potentials are to be compared are connected with the two pairs of quadrants. If A and B be their potentials, and C the potential of the needle, it may be shown (see Maxwell, Electricity and Magnetism, § 219) that the couple tending to turn the needle from A to B is

a(A-B) {C-(A+B)} . . . . . (6), where a is a constant depending on the dimensions of the instru ment. If C be very great compared with (A + B), as it usually is, then the couple is

aC(A-B).

(7) simply; in other words, the couple varies as the difference between the potentials of the quadrants. Some idea of the general distribution of the parts of the actual instrument may be gathered from fig. 15, which gives an elevation and a section of the instrument. The case forms a Leyden jar as usual in Thomson's electrometers; the internal coating in this instance is formed by a quantity of concentrated sulphuric acid, which also keeps the inside of the instrument dry. The quadrants are suspended by glass pillars from the lid of the jar, and one of these pillars is supported on a sliding piece, arranged on strict kinematical principles, so as to be movable in a horizontal direction by means of a micrometer screw Y. This motion is used to adjust the position of the needle, when charged, so that its axis may fall exactly between the quadrants A, C, and B, D. A glass stem C, rising from the lid of the jar into a superstructure called the "lantern," supports a metal piece Z, to which is fastened a metal framework fitted with supports and adjustments for the bifilar suspension of the needle. A fine platinum wire drops from the needle into the sulphuric acid, thus connecting the needle with the inside coating of the jar. This tail wire is also furnished with a vane, which works in the acid and damps the oscillations of the needle. A stout aluminium wire rises from the needle, carries a light concave mirror T, and ends in a cross piece to which are attached the suspension fibres. The aluminium stem and the platinum tail wire are defended from electrical disturbances by a guard tube, which is in metallic connection with tho piece Z, and also, by means of a platinum wire, with the acid; it is through this, by means of the "temporary electrode" P, that the inside of the jar is charged. The two principal electrodes are P and M. Connected with Z is a metal disc S, attracting the aluminium balance of a gauge like that of the absolute electrometer. This gauge is well seen in the bird's-eye view given in fig. 16. A

O

FIG. 16.-Thomson's Quadrant Electrometer-Bird's-eye view. replenisher, like that in the absolute electrometer, is fitted to the lid of the jar, and by means of it the potential of the needle can be adjusted till the hair of the guage is in the sighted position.

The deflections of the instrument are read off by means of an image formed by the mirror T on a scale at the distance of a metre or so, the object being a wire which is stretched below the scale in a slit illuminated by a lamp. Within certain limits the deflections are proportional to the deflecting couple, i.e., to the difference between the potentials of the quadrants A, D and B, C; but where this is mot so, the instrument can easily be graduated experimentally.

For many purposes, especially in the lecture room, an instrument

so complicated as the above is unnecessary and undesirable. A simpler form (fig. 17) of quadrant electrometer is now manufactured by Elliot Brothers, and answers most ordinary purposes very well. Capillary Electrometers.-Electrometers have recently been constructed by taking advantage of the fact that the surface tension

of mercury is greatly affected by the hydrogen deposited on it when it is the negative electrode in contact with dilute sulphuric acid (see ELECTROLYSIS, p. 109). A quantity of mercury is placed in the bottom of a test tube, and communicates with a platinum electrode let in through the bottom of the tube; on the mereury is poured dilute sulphuric acid, and into this dips a tube drawn out into a capillary ending. This tube contains mercury down to a certain mark on the capillary part, FIG. 17.-Quadrant Electrometer. the remainder being occupied

with acid which is continuous with that in the test tube. So long as the mercury in the test tube is simply in metallic connection with that in the upper tube, the position of the mercury in the capillary part is stationary; but if an electromotive force be introduced into the external circuit, acting towards the test tube, then hydrogen is deposited on the small mercury surface, its surface tension increases, and the pressure in the tube must be considerably increased to maintain the mercury at the mark. This increase of pressure is proportional to the electromotive force within certain limits, hence we can use this arrangement as an electrometer. Electrometric Measurement.-Several examples of électrometric measurement will be found in the article ELECTRICITY (pp. 18, 37, 88, 42, 46, &c.). We recommend in this connection the study of the sections on atmospheric electricity in Sir Wm. Thomson's Reprint of Papers on Electricity and Magnetism, and sections 220 and 229 in Clerk Maxwell's Electricity and Magnetism. We have been drawing throughout on Thomson's Report on Electrometers and Electrometric Measurements, but it will not be amiss to draw attention to it once more. (G. CH.)

ELEMI. The resin thus termed in modern pharmacy is obtained by incising the trunk of a species of Canarium found in the Philippine Islands. It is a soft, more or less translucent, adhesive substance, of granular consistency and fennel-like smell, and colourless when pure, but sometimes grey or blackish from the presence of carbonaceous and other impurities. When exposed to the air it becomes yellowish in tint, and harder. It consists mainly of essential oil, and of an amorphous and a crystalline resin, the former easily soluble in cold, and the latter only in hot alcohol. Elemi is used chiefly in the manufacture of spirit and turpentine varnishes, which it enables to dry without cracking. As a constituent of a stimulating ointment, it has found a place in British pharmacopoeias. In the Philippines it is employed for caulking ships, and is kneaded with rice-husks for torches (see Jagor, Reisen in den Philippinen, p. 79, Berlin, 1873). The word elemi, like the older term animi, appears to have been derived from enhamon (Greek, vapor), the name of a styptic medicine said by Pliny to contain tears exuded by the olive-tree of Arabia. This tree, according to Flückiger and Hanbury, is probably to be identified with the Boswellia Frereana of Birdwood, which flourishes in the neighbourhood of Bunder Marayah, west of Cape Gardafui (see S. B. Miles, Journ. R. Geog. Soc., xlii. p. 64). Mexican or Vera Cruz elemi, formerly imported into England, is afforded by the species Amyris elemifera, Royle; Mauritius elemi by another tree, Colophonia Mauritiana, D.C.; and Brazilian elemi by several species of Icica. For a paper "On the Chemistry of Elemi," see Flückiger, Year-Book of Pharmacy, 1874, p. 496.

ELEPHANT (Elephantida), a family of pachydermatous mammals belonging to the order Proboscidea, contair ng only a single existing genus and two species-the sole sur viving representatives of the entire order. The elephants are characterized by great massiveness of body, constituting

Lapp mann' capillary electremeter.