Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/132

experimcnts in which Hankel used it are alluded to in the article Electricity.

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 in- sulated quadrants A, B, C, D. The opposite airs A, D and B, C are connected by thin platinum wires. The two odies whose potentials are to be compared are connected with the two pairs of uadrants. If A and B be their potentials, and C the potential of t e needle, it may be shown (see Maxwell, Electricity and Magnetism, § 219) that the couple tending to turn the needle from A to B is

dA—BHC—MA+Bfl .. .. . at

where a is a constant depending on the dimensions of the instru- ment. If C be very great compared with 13(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 distribu- tion 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 quad- rants 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 bililar 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, aml ends in a cross piece to which are attached the suspension fibres. The aluminium stem and the platinum tail wire are defended from electrical disturb- ances by a guard tube, which is in metallic connection with the piece Z, and also, by means of a platinum wire, with the acid ; it is through this, by means of the “ temporary clcctrodc " 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 alu- minium 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 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.

FIG. 16.—Thomson's Quadrant Electrometer—Bird's-eye view.

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. \Vithin 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 not so, the instrument can easily he graduated experimentally.

For many purposes, especi.lly 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.—Elcc- trometers have recently been con- structed 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 a 77 certain mark on the ca illa art .— ,. ' F , the remainder being Oglfifieé FIG. 11.—Quadrant blectl'ometer. 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 clectromotive force be intro- duced 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 electrometric measurement will be found in the article Electricity (pp. 18, 37, 38, 43, 46, &c.). We recommend in this connection the study of the sections on atmospheric electricity in Sir Wm. Thomson's 11’6- print of Papers on Electricity and Illagnctimn, and sections 220 and 229 in Clerk Maxwell's Electricity and Magnetism. We have been drawing throughout on Thomson’s Report on Electromelcrs and Elccli'omelric Alcasurcmcnts, 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. Uunarium 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 essen— tial 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, It’cism in den Philippinen, p. 79, Berlin, 1873). The word elemi, like the older term animi, appears to have been derived from enhwmon (Greek, é’vaquov), the name of a styptic medicine said by Pliny to contain tears exuded by the olive—tree of Arabia. This tree, according to F liiekiger and Hanbury, is probably to be identified with the Boswell ia Frereana of Birdwood, which flourishes in the neighbour- hood of Bunder Marayah, west of Cape Gardafui (see S. B. Miles, Journ. 11’. Geog. Soc.,xlii. p. 64). Mexican or Vera Cruz elemi, formerly imported into England, is afforded by the species Amy/H's elemifera, Royle ; Mauritius elemi by another tree, Coloplionia Jllauriliana, DC; and Brazilian elemi by several species of Icica. For a paper “ On the Chemistry of Elemi,” see Fliickiger, Year—Book of Pharmacy, 1874, p. 496.

ELEPHANT (Elephanlidcc), a family of pachydermatous

mammals belonging to the order Proboscidea, containing only a single existing genus and two species—the sole surviving representatives of the entire order. The elephants

are characterized by great massiveness of body, constituting