species of Cynips; in August they become detached from the leaves that bear them, and are caused to jump by the spasmodic movements of the grub within the thin-walled gall-cavity.1

a

d

F. 1.-a, Aleppo "blue" gall; b, ditto in section, showing central cavity for grub; Aleppo "white" gall, perforated by insect; d, the same in section Datural size).

Common gall-nuts, nut-galls, or oak-galls, the Aleppo, Turkey, or Levant galls of commerce (German, Galläpfel, Levantische Gallen; French, Noix de Galle), are produced on Quercus infectoria, a variety of Q. Lusitanica, Webb, by Cynips (Diplolepis, Latr.) tinctoria, L., or C. galla tinctoria, Oliv. Aleppo galls (galla halepenses) are brittle, hard, spherical bodies, --inch in diameter, ridged and warty on the upper half, and light brown What are termed blue, to dark greyish-yellow within. black," or green" galls contain the insect; the inferior "white" galls, which are lighter coloured, and not so compact, heavy, or astringent, are gathered after its escape (see fig. 1). Less valued are the galls of Tripoli (Taraplus or Tarabulus, whence the name "Tarabious galls"). The most esteemed Syrian galls, according to Pereira, are those of Mosul on the Tigris. Other varieties of nut-galls, besides the above mentioned, are employed in Europe for various purposes. Commercial gall-nuts have yielded on analysis from 26 (H. Davy) to 77 (Buchner) per cent. of tannin (see Vinen, loc. cit.), with gallic and ellagic acids, ligueous fibre, water, and minute quantities of proteids, chlorophyll, resin, free sugar, and, in the cells around the inner shelly chamber, calcium oxalate. Oak-galls are mentioned by Theophrastus, Dioscorides (i. 146), and other ancient writers, including Pliny (Nat. Hist., xvi. 9, 10; xxiv. 5), according to whom they may be produced "in a single night.' Their insect origin appears to have been entirely unsuspected until within comparatively recent times, though Pliny, indeed, makes the observation that a kind of gnat is produced in certain excrescences on oak leaves. Bacon describes cak-apples as "an exudation of plants joined with putrefaction." Pomet thought that gall-nuts were the fruit of the oak, and a similar opinion obtains among the modern Chinese, who apply to them the term Mu-shih-tsze, or "fruits for the foodless."3 Hippoerates administered gall-nuts for their astringent properties, and Pliny (Nat. Hist., xxiv. 5) recommends them as a remedy in affections of the gums and uvula, ulcerations of the mouth, and some dozen more complaints. The drug has been used in the treatment of intermittent fevers, but appears to be adapted only for their mildest phases. In India it is given also in chronic diarrhea, dysentery, gonorrhoea, and several other diseases. In Pritish pharmacy gall-nuts are used in the preparation of the two astringent ointments unguentum galle and unguentum gallæ cum opio, and of the tinctura galle, and also as a source of tannin and of gallic acid (q.v.). They have from very early times been resorted to as a means of staining the hair of a dark colour, and they are the base of the tattooing dye of the Somali women." On the Continent they are employed in tanning. With respect to the echnical application of gall-nuts, see further BLASTING, vol. iii. p. 808, DYEING, vol. vii. p. 579, and INK. In consequence of the increased consumption in dyeing of sumach, myrobalans, and new chemical sub

1 See R. H. Stretch and C. D. Gibbes, Proc. California Acad. Nat. Sciences, iv. pp. 265 and 266.

A Complete History of Drugs (translation), p. 169, Lond., 1748. F. Porter Smith, Contrib. towards the Mat. Medica... of China, p. 100. 1871. Cullen, Mat. Med., ii. p. 46, 1789.

E. J. Waring, Pharm. of India, p. 463, 1868.

E. J. Waring, Remarks on... Bazaar Medicines... of India, Lond., 3d ed., 1875.

R. F. Burton, First Footsteps in E. Africa, p. 178, 1856.

stances, the British importations of gall-nuts have on the whole declined considerably.

The quantities and values of galls imported into the United Kingdon in 1877 were as follows:-From Germany, 1963 cwts., £7759; Turkey, 6420 cwts., £20,712; Egypt, 1702 cwts., £6244: China, 11,748 cwts., £32,715; British India (Bombay and Scinde), 2181 cwts., £2230; other countries, 2411 cwts., £7176; total, 26,425 cwts., of the value of £76,834, against 25,884 cwts., value £64,704, in 1876.

The gall-making Hymenoptera include, besides the Cynipide proper, certain species of the genus Eurytoma (Isosoma, Walsh) and family Chalcidida, e.g., E. hordei, the "joint-worm" of the United States, which produces galls on the stalks of wheat;8 also various members of the family Tenthredinida, or saw-flies. The larvæ of the latter usually vacate their galls to spin their cocoons in the earth, or, as in the case of Athalia abdominalis, Klg., of the clematis, may emerge from their shelter to feed for some days on the leaves of the gall-bearing plant.

The dipterous gall-formers include the gall-midges, or gall-gnats (Cecidomyidae), minute slender-bodied insects, with bodies usually covered with long hairs, and the wings folded over the back. Some of them build cocoons within their galls, others descend to the ground to become pupæ. The true willow-galls are the work either of these or of sawflies. Their galls are to be met with on a great variety of plants of widely distinct genera, e.g., the ash, maple, hornbeam, oak, grape-vine, 10 alder, gooseberry, blackberry, pine, juniper, thistle, fennel, meadowsweet,11 common cabbage, and cereals. In the northern United States, in May, "legions of these delicate minute flies fill the air at twilight, hovering over wheat-fields and shrubbery. A strong northwest wind, at such times, is of incalculable value to the farmer."12 Other gall-making dipterous flies are members of the family Trypetida, which disfigure the seed-heads of plants, and of the family Mycetophilidæ, such as the species Sciara tilicola, 13 Löw, the cause of the oblong or rounded green and red galls of the young shoots and leaves of the lime.

Galls are formed also by hemipterous and homopterous insects of the families Tingida, Psyllida, Coccide, and Aphida. Coccus pinicorticis causes the growth of patches of white flocculent and downy matter on the smooth bark The galls of young trees of the white pine in America. 14 of examples of the last family are common objects on limeleaves, and on the petioles, of the poplar. An American Aphid of the genus Pemphigus produces black, ragged, leathery, and cup-shaped excrescences branches of the hickory.

on the young

The Chinese galls of commerce (Woo-pei-tsze) are stated to be produced by Aphis Chinensis, Bell, on Rhus semialata, Murr. (R. Bucki-amela, Roxb.), an Anacardiaceous tree indigenous to N. India, China, and Japan. They are hollow, brittle, irregularly pyriform, tuberculated or branched vesicles, with thin walls, covered externally with a grey down, and internally with a white chalk-like matter, and insect-remains (see fig. 2). The escape of the insect takes place on the spontaneous bursting of the walls of the vesicle, probably when, after viviparous (thelytokous) reproduction for several generations, male winged insects are developed. The galls

8 A. S. Packard, jun., Guide to the Study of Insects, p. 205, Salem, 1870. 9 On the Cecidomyids of Quercus Cerris, see Fitch, Entomologist, xi. p. 14.

10 See, on Cecidomyia oenephila, Von Haimhoffen, Verhandl. d. zoolog.-bot. Ges. in Wien, xxv., 801-10.

11 See Entomologist's Month. Mag., iv., 1868, p. 233; and for figure and description, Entomologist, xi. p. 13.

12 A. S. Packard, jun., Our Common Insects, p. 203, Salem, U.S., 1873. On the Hessian fly, Cecidomyia destructor, Say, the May brood of which produces swellings immediately above the joints of barley attacked by it, see Asa Fitch, The Hessian Fly, Albany, 1847, reprinted from Trans. New York State Agric. Soc., vol. vi.

13 J. Winnertz, Beitrag zu einer Monographie der Seiarinen, p. 164, Vienna, 1867.

14 Asa Fitch, First and Second Rep. on the Noxious.... Insects of the State of New York, p. 167, Albany, 1856.

are gathered before the frosts set in, and are exposed to steam to kill the insects.1 Chinese galls examined by Viedt yielded 72 per cent. of tannin, and less mucilage than Aleppo galls. Several other varieties of galls are produced by Aphides on species of Pistacia.

M. J. Lichtenstein has established the fact that from the egg of the Aphis of Pistachio galls, Anopleura lentisci, is hatched an apterous insect (the gall-founder), which gives birth to young Aphides (emigrants), and that these, having acquired wings, fly to the roots of certain grasses (Bromus sterilis and Hordeum vulgare), and by budding underground give rise to several generations of

秒速度

a

FIG. 2.-a, Chinese gali (half natural size), b, ditto, broken, showing thin-walled cavity; c, Japanese gall (natural size).

apterous insects, whence finally comes a winged brood (the pupifera). These last issuing from the ground fly to the Pistachio, and on it deposit their pupa. From the pupa, again, are developed sexual individuals, the females of which lay fecundated eggs productive of gall-founders, thus recommencing the biological cycle (see Compt. Rend., Nov. 18, 1878, p. 782, quoted in Ann. and Mag. Nat. Hist., 1879, p. 174).

Of other insects which have been recognized as gall makers there are, among the Coleoptera, certain Curculionids (gall-weevils), and species of the exotic Sagride and Lamiada, and an American beetle, Saperda inornata (Cerambycidae), which forms the pseudo-galls of Salix longifolia and Populus angulata, or cottonwood. Among the Lepidoptera are gall-forming species belonging to the Tineida, Egeriida, Tortricidae, and Pterophorida. The larva of a New Zealand moth, Morova subfasciata, Walk. (Cacoëcia gallicolens), of the family Drepanulidæ, causes the stem of a creeping plant, on the pith of which it apparently subsists, to swell up into a fusiform gall.3

Mite galls, or acarocecidia, are abnormal growths of the leaves of plants, produced by microscopic Acaridea of the genus Phytoptus (gall-mites), and consist of little tufts of hairs, or of thickened portions of the leaves, usually most hypertrophied on the upper surface, so that the lower is drawn up into the interior, producing a bursiform cavity. Mite-galls occur on the sycamore, pear, plum, ash, alder, vine, mulberry, and many other plants; and formerly, e.g., the gall known as Erineum quercinum, on the leaves of Quercus Cerris, were taken for cryptogamic structures. The lime-leaf "nail-galls" of Phytoptus tiliae closely resemble the "trumpetgalls" formed on American vines by a species of Cecidomyia. Certain minute Nematoid worms, as Anguillula scandens, which infests the ears of wheat, also give rise to galls.

Besides the larva of the gall-maker, or the householder, galls usually contain inquilines or lodgers, the larvæ of what

See E. Doubleday, Pharm. Journ., 1st ser., vol. vii. p. 310; and Pereira, ib., vol. iii. p. 377.

Dingler's Polyt. Journ., ccxvi. p. 453; cf. supra GALLIC ACID. For figure and description see Zoology of the Erebus and Terror, ii. pp. 46, 47, 1844-75.

On the mite-galls and their makers, see F. Löw, "Beiträge zur Naturgesch. der Gallmilben (Phytoptus, Duj.)," Verhandl. d. zoolog.. bot. Ges. in Wien, xxiv., 1874, pp. 2-16, with plate; and "Ueber Milbengallen (Acarocecidien) der Wiener-Gegend," ib., pp. 495-508; Andrew Murray, Economic Entomology, Aptera, pp. 331-374, 1876; and F. A. W. Thomas, Aeltore und neue Beobachtungen über PhytoptoCecidien, Halle, 1877.

are termed guest-flies or cuckoo-flies. Thus the galls of Cynips and its allies are inhabited by members of other cynipideous genera, as Synergus, Amblynotus, and Synophrus; and the pine-cone-like gall of Salix strobiloides, as Walsh has shown," is made by a large species of Cecidomyia, which inhabits the heart of the mass, the numerous smaller cecidomyidous larvæ in its outer part being mere inquilines. In many instances the lodgers are not of the same order of insects as the gall-makers. Some saw-flies, for example, are inquilinous in the galls of gall-gnats, and some gall-gnats in the galls of saw-flies. Again, galls may afford harbour to insects which are not essentially gall-feeders, as in the case of the Curculio beetle Conotrachelius nenuphar, Hbst., of which one brood eats the fleshy part of the plum and peach, and another lives in the "black knot" of the plum-tree, regarded by Walsh as probably a true cecidomyidous gall. The same authority (loc. cit., p. 550) mentions a willow-gall which provides no less than sixteen insects with food and protection; these are preyed upon by about eight others, so that altogether some twenty-four insects, representing eight orders, are dependent for their existence on what to the common observer appears to be nothing but "an unmeaning mass of leaves." Among the numerous insects parasitic on the inhabitants of galls are hymenopterous flies of the family Proctotrypida, and of the family Chalcidida, e.g., Callimome regius, the larva of which preys on the larvae of both Cynips glutinosa and its lodger Synergus facialis. The oak-apple which Von Schlechtendal (loc. sup. cit., p. 33) considers to often contains the larvae of Braconida and Ichneumonidæ, be parasites not on the owner of the gall, Andricus terminalis, but on inquilinous Tortricidae. Birds are to be included among the enemies of gall-insects. Oak-galls, for example, are broken open by the titmouse in order to obtain the grub within, and the "button-galls" of Neuroterus numismatis, Oliv., are eaten by pheasants.

On galls and their makers and inhabitants see further-J. T. C. Ratzeburg, Die Forst-Insecten, Th. iii. pp. 53 sq., Berlin, 1844; T. W. Harris, Insects injurious to Vegetation, Boston, U.S., 2d ed., 1852; C. L. Koch, Die Pflanzenläuse Aphiden, Nuremberg, 1854; T. Hartig, Die Familien der Blatt wespen und Holzwespen, Berlin, 1860; Walsh, "On the Insects, Coleopterous, Hymenopterous, and Dipterous, inhabiting the Galls of certain species of Willow," Proc. Ent. Soc. Philadelphia, iii., 1863-4, pp. 543-644, and vi., 1866-7, pp. 223-288; T. A. Marshall, "On some British Cynipide," Ent. Month. Mag., iv. pp. 6-8, &c.; H. W. Kidd and Albert Müller, "A List of Gall-Bearing British Plants," ib., v. pp. 118 and 216; G. L. Mayr, Die mitteleuropäischen Eichengallen in Wort und Bild, Vienna, 1870-71, and the translation of that work, with notes, in the Entomologist, vols. vii. sq.; also, by the same author, "Die Einmiethler der mitteleuropäischen Eichengallen," Verhandl. d. zoolog.. Torymiden," ib., xxiv. pp. 53-142 (abstracted in Cistula Entomobot. Ges. in Wien, xxii. pp. 669-726; and "Die europäischen logica, i., Lond., 1869-76); F. Löw," Beiträge zur Kenntniss der Gallmücken," ib., pp. 143-162, and 321-328; J. E. von Bergenstamm and P. Low, "Synopsis Cecidomyidar am," ib., xxvi. pp. 1-104; Perris, Ann. Soc. Entom. de France, 4th ser., vol. x. pp. 176-185; R. Osten-Sacken, "On the North American Cecidomyide,' Smithsonian Miscellaneous Collections, vol. vi., 1867, p. 173; E. L. Taschenberg, Entomologie für Gärtner und Gartenfreunden, Leipsic, 1871; J. W. H. Traill, "Scottish Galls," Scottish Naturalist, i, 1871, pp. 123, &c.; Albert Müller, "British Gall Insects," The Entomologist's Annual for 1872, pp. 1-22; B. Altum, Forstzoologie, iii., "Insecten," pp. 250 sq., Berlin, 1874; J. H. Kaltenbach, Die Planzen Feinde aus der Classe der Insecten, Stuttg., 1874; A. d'Arbois de Jubainville and J. Vesque, Les Maladies des Plantes Cultivées, pp. 98-105, Paris, 1878. (F. H. B.)

GALLUPPI, PASQUALE (1770-1846), a distinguished Italian philosopher, was born on 2d April 1770, at Tropea, in Calabria. He was of good family, and after completing his education at the academy of Tropea and the university of Naples he entered the public service, and was for many years employed in the office of the administration of finances. Altogether apart from academic influences he pursued his favourite studies, and it was not till he had reached the age

Proc. Entomol. Soc. Philadelphia, iii., 1864, p. 549.

of sixty, and had become widely known by his writings on philosophy, that he was called to a chair in the university of Naples. This chair he held till his death in November 1846. Galluppi's first work was an essay on analysis and synthesis (Sull' Analisa e sulla Sintesi) published in 1807. This was followed by the important Saggio filosofico sulla critica della conoscenza, in 6 volumes, published from 1819 to 1832. In the Lettere filosofiche sulle vicende della filosofia relativamente ai principii delle conoscenze humana, da Cartesio sino a Kant, 1827, by which, through the translation into French (by M. Peisse, 1844), he is best known to foreigners, Galluppi traces his own philosophical development from the empiricism of the 18th century writers through the Kantian criticism to his final speculative view, one in many respects resembling the doctrines of the Scotch school as amended by Hamilton. His systematic work, Elementi di filosofia (4 vols. 1832), was long used as a textbook for instruction in the Italian colleges. Of other writings may be mentioned the Lezioni di logica e di metafisica (1832-3, 5 vols., 1842); the Filosofia della Volonta (3 vols., 1832-1842, incomplete); and the Storia della Filosofia (1842), of which only the first volume was published. Galluppi, though in many respects Kantian, can hardly be said to have taken up fully the speculative significance of the Critique of Pure Reason. He accepts the Kantian demonstration of the necessary unity of consciousness as the indispensable factor in knowledge, regards our knowledge of the ego as knowledge of substance, maintains that in external perception, or, as he puts it, in sensation, we are directly cognizant of the real thing, and holds that the existence of the unconditioned is given in knowledge as the necessary correlate of the conditioned, but rejects entirely the a priori element which is the distinguishing characteristic of the Kantian doctrine of cognition. All judgments, according to him, are ultimately identical-a relic of the empiricism of Condillac which is totally irreconcilable with the fundamental principles of his philosophy. On the other hand, Galluppi exaggerates the place and importance of the moral reason; with Kant he finds objective truth in the ideas of desert and duty, and admits that ethical judgments are a priori, without endeavouring to explain, in accordance with his theoretical views. how such judgments are at all possible.

A good view of Galluppi's place in Italian philosopny is given in Ferri, Essai sur l'Histoire de la Phil. en Italie au XIXme Siècle, vol. i, Paris, 1869. See also V. Botta, in Ueberweg's History of Phil. (Eng. transl., vol. ii., appendix ii.); Prof. Barzellotti, "Philosophy in Italy," in Mind, October 1878.

GALLUS, C. CORNELIUS, a Roman poet, orator, and politician, was born of humble parents at Forum Julii (Fréjus), in Gaul, about the year 66 B.C. At an early age he removed to Rome, where he was taught by the same master as Virgil and Varius. In political life he espoused the cause of Octavianus, and as a reward for his services was made prefect of Egypt. His conduct in this position afterwards brought him into disgrace with Augustus, and dreading the exposure of his arrogance, extortion, and cruelty, he put an end to his life by throwing himself on his sword, in the year 26 B.C. Gallus enjoyed a high reputation among his contemporaries as a man of intellect. He associated on terms of equality with Virgil, Ovid, Varins, Asinius Pollio, and others, and on account of his four books of elegies Ovid claimed for him the first place among the elegiac poets of Rome. His fame as an orator was hardly inferior to his renown as a poet; but as not a fragment of his composition has descended to our times, we have no means of judging the worth of his literary pretensions, and have to content ourselves with the somewhat partial estimate of his personal friends.

Sea Ch. C. Völker, Commentatio de C. Galli Vita et Scriptis, part L. Bonn, 1840; part ii, Elberfeld, 1844.

GALT, JOHN (1779-1839), a Scottish novelist, was born at Irvine, in Ayrshire, on May 2, 1779. He received his early education at Irvine and Greenock, and read largely from one of the public libraries while serving as a clerk in a mercantile office. His first compositions appeared in the Greenock Advertiser and the Scots Magazine. In 1804 he' went to settle in London, where he continued to work at a poem on the Battle of Largs, which was published anonymously. After unsuccessful attempts to succeed in business, Galt left for the Continent, and met Byron and Sir John Hobhouse at Gibraltar, with whom he had a tour in the Mediterranean. He remained abroad for three years, and then returned to London. His early works are the Life and Administration of Wolsey, Voyages and Travels, Letters from the Levant, the Life of Benjamin West, Historical Pictures, the Wandering Jew, and a volume of dramas; but he first showed his real power in The Ayrshire Legatees, which appeared in Blackwood's Magazine in 1820. This was followed in 1821 by his masterpiece-The Annals of the Parish; and, at short intervals, Sir Andrew Wylie, The Entail, The Steam-Boat, and The Provost were published. These are all in his happiest manner, and are unsurpassed as studies of Scottish character. His next works were Ringan Gilhaize, a story of the Covenanters; The Spaewife, which relates to the times of James I. of Scotland; Rothelan, a novel founded on the reign of Edward III.; The Omen, which was favourably criticized by Sir Walter Scott; and The Last of the Lairds, another picture of Scottish life. In 1826 he visited America for the second time, in connexion with the establishment of the Canada Company—an undertaking which involved him in great difficulties, and ultimately proved disastrous to his worldly prospects. It is pleasant to remember that, although Galt's connexion with Canada was unfortuate for himself, his youngest son, Sir Alexander Galt, has had a distinguished career there, and was, for some time, finance minister of the colony. In 1827 Galt founded Guelph in Upper Canada, passing on his way the township of Galt on the Grand River, named after him by the Hon. William Dixon. In 1829 he returned to England commercially a ruined man, and devoted himself with great ardour to literary pursuits, of which the first fruit was Lawrie Todd-one of his best novels. Then came Southennan, a tale of Scottish life in the times of Queen Mary. In 1830 he was appointed editor of the Courier newspaper-a post he soon relinquished. His untiring industry was seen in the publication, in rapid succession, of a Life of Byron, Lives of the Players, Bogle Corbet, Stanley Buxton, The Member, The Radical, Eben Erskine, The Stolen Child, his Autobiography, and a collec tion of tales entitled Stories of the Study. In 1834 appeared his Literary Life and Miscellanies, dedicated by permission to William IV., who sent the author a present of £200. As soon as this work was published Galt retired to Greenock, where he lingered on in bad health till his death on the 11th of April 1839.

Galt, like almost all voluminous writers, was excecaingly unequal. His masterpieces are The Ayrshire Legatees, The Annals of the Parish, Sir Andrew Wylie, The Entail, The Provost, and Lawrie Todd. The Ayrshire Legatees gives, in the form of a number of exceedingly diverting letters, the adventures of the Rev. Dr Pringle and his family in

London. The letters are made the excuse for endless teaparties and meetings of kirk session in the rural parish of Garnock. The Annals of the Parish are told by the Rev. Micah Balwhidder, Galt's finest character. This work is a splendid picture of the old-fashioned Scottish pastor and the life of a country parish; and, in rich humour, genuine' pathos, and truth to nature, it is unsurpassed even by Scott. Like his other Scotch novels, it is a fine specimen of the much homely graces of the Scottish dialect, and preserves

recorded the observation that muscular contractions may be caused in a prepared frog merely by bending back the legs and bringing them into contact with the lumbar nerves, as also when a nerve is touched at two different points with a morsel of muscle taken from a living frog, phenomena not satisfactorily explicable on the theories of Volta; but after the death of the Bologna professor very little was heard of animal electricity till, in 1827, the study of the subject was resumed by Nobili. On Galvani's refusal, from religious scruples, to take the oath of allegiance to the Cisalpine republic on its establishment, he was removed from his fessorship. Deprived thus of the means of livelihood, he retired to the house of his brother Giacomo, where he soon fell into a feverish decline. The republican Government, in consideration of his great scientific fame, eventually, but too late, determined to reinstate him in his chair at the university of Bologna. He died December 4, 1798. A quarto edition of his works was published at Bologna in 1841-42, by the Academy of Sciences of the Institute of that city, under the title Opere edite ed inedite del professore Luigi Galvani.

pro

See Volta, "An Account of some Discoveries made by Mr Galvani, of Bologna," in Phil. Trans., 1793, pp. 10-44; J. L. Alibert, Elogio Storico di Luigi Galvani, Traduzione dal Francese, Bolog, 1802, fol.; Arago, in "Alexandre Volta," Euvres Complètes, ed. Barral, t. i. p. 242, 1851; and H. M. Noad, Manual of Electricity, chap. x.; also ELECTRICITY, vol. viii. p. 9, col. 1, and VOLTA.

GALVANISM. See ELECTRICITY and PHYSIOLOGY.

GALVANOMETER, an instrument used for indicating or measuring currents of electricity, wherein advantage is taken of the force exerted by such currents on movable magnets in their neighbourhood. When a galvanometer is used for indicating merely, without measuring, it is sometimes called a galvanoscope. If we consider only such instruments as have come into actual use, this definition is strict enough for practical purposes. instruments that have been or might be made, some would

If we

were to consider all the

come under the definition whose resemblance to the modern

pans

galvanometer would not at first sight be apparent. Such, for instance, is the electromagnetic balance of Becquerel, which consists of two bar magnets hung from the scale of a delicate balance each in the axis of a cylindrical bobbin of wire-one being over, the other under its corresponding bobbin (see fig. 1). The north poles of both magnets hang

Fig. 1.

downwards, and the current to be measured is sent round the bobbin, so that each of the magnets is repelled. Weights are put into the left-hand scale until equilibrium in the original position is restored. The weight thus added is proportional to the current strength, so long as the induced magnetism of the magnets can be neglected. This instrument has fallen into disuse.

In a complete galvanometer of modern construction the following parts may occur: (1) the coil or multiplier, (2)

For another definition see the article ELECTROMETER. Î

For a brief history of the construction of galvanometric apparatus ELECTRICITY, vol. viii. p. 13.

10-4

the needle or movable magnet or magnets, (3) the astatizing apparatus, (4) the deflecting or adjusting magnet, (5) the graduation or reading apparatus, (6) the damping apparatus, (7) accompanying the galvanometer, as a piece of auxiliary apparatus, we may also have a box of shunts. It would be easy to make a more minute enumeration of parts, but the above will serve our present purpose. On the other hand, it is not always that each of the above organs is represented separately; some may be wanting in certain cases, and the functions of two or more may be combined.

1. The multiplier or coil consists of a ring-shaped channel of elliptical, rectangular, or circular shape-usually the last, the cross section being in general rectangular. Into this is wound, as closely and regularly as possible, a quantity of silk-covered wire. The material chosen for the wire is usually copper, which should be as soft as possible in order to secure high conductivity.. White silk is preferred for the insulating covering, on account of its freedom from iron, though this is for most purposes a needless refinement. Great care should be taken that the wire is dry when it is wound. It is usual, in order to secure and render permanent the insulation, to steep the whole coil in melted paraffin ; after this has been done, there is little risk of loss of insulation, provided the layers have been carefully tested during the winding. The idea of the multiplier in sensitive galvanometers is to bring the greatest number of coils of wire within the least possible distances of the magnet. It is evident, therefore, that the insulating covering should be as thin as is consistent with good insulation; this considerahave to be wound. After the wire has reached a certain tion assumes great importance when coils of very fine wire fineness the proportion of space occupied by insulating matter is so great that further reduction of the section of the wire simply increases the resistance without enabling us to pack more turns into the same space. In general the section of the wire ought to be chosen with reference to the use which the galvanometer is intended to serve. following ideal case will enable the reader to comprehend the principle which regulates the choice of multiplier under given circumstances. Suppose the dimensions of the channel, and the whole space which the wire is to fill, to be given, and the whole external resistance also given, then it may be shown that the section of the wire ought to be chosen so that the resistance of the galvanometer shall be equal to the external resistance. The case contemplated here is that where we have a simple external circuit; many cases can be reduced to this at once, and we shall consider below a more complicated case of considerable practical importance. Theoretically the section of the wire ought to vary with the distance of the winding from the axis of the coil. The law is that the diameter of the wire in each layer ought to be proportional to the linear dimension of that layer. This is sometimes roughly carried out in practice by winding the outer layers of thicker wire than the inner. The proper form of the longitudinal section of the coil depends on the use for which the instrument is destined, and will be more properly discussed when we describe particular instruments. In a certain class of galvanometers called differential, the wire on the coil is wound double, so that two currents can be sent through side by side in the same or in opposite directions.

is

The

2. The needle consists of a piece of magnetized steel,

supposed to vary in thickness as the diameter of the wire.

* In this and all that follows the silk covering is either neglected or

The cross section of the coil is not a matter of indifference in

sensitive galvanometers; but the question is hardly of sufficient importance to need discussion here. Information on the subject

will be found in W. Weber's Electrodynamische Mansbestimmungen, Thl. ii.; H. Weber, Pogg. Ann., 1869; Maxwell's Electricity and Magnetism, vol. ii. secs, 716 22: Jenkin's Electricity and Magi

netisme, cap. xiii. sec.