Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/59

recorded the observation that muscular contractions may be caused in a prepared frog Inerer 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 Ihe 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 Galvaui’s refusal, from religious scruples, to take the oath of allegiance to the Cisalpine re- public on its establishment, he was removed from his pro- 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. Tile 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 18411—42, by the Academy of Sciences of the Institute of that city, under the title ()perc editc ed inedite deljn‘ofessore

Luigi Galvani.

See Volta, “ An Account of some Discoveries made by Mr Galvani, of Bologna," in Phil. T1°lt7l3., 1793, pp. 10—44; J. L. Alibert, Elogio Storico di Luigi Galvmu', Traduzionc dal Franccsc, Bolog., 1802, fol.; Arago, in “Alexandre Volta,” (Elwrcs Complctcs, ed. Barral, t. i. p. 242, 1854; and }l. M. Noad, Jlanual 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.^[1] 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. If we were to consider all the instruments that have been or might be made, some would come under the definition whose resemblance to the modern galvanometer would not at first sight be apparent. Such, for instance, is the electromagnetic balance of Becquerel,^[2] which consists of two bar magnets hung from the scale pans 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 downwards, and the current to be measured is sent round ‘. the bobbin, so that each of the magnets is repelled. i Weights are put into the left—hand scale until equilibrium l 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.

Fig. 1.

In a complete galvanometer of modern construction the following parts may occur :—(1) the coil or multiplier, (2) the needle or movable magnet or magnets, 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 preferer for the insulating covering, on account of its freedom from iron, though this is for most purposes aneedless refinement. Great care should be taken that the wire is dry when it is wound. It is usual, in order to secure and render perman- ent the insulation, to steep the whole coil in melted paraffin ; after this has been done, there is little risk of loss of insula- tion, 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 considera- tion assumes great importance when coils of very fine wire have to be wound. After the wire has reached a certain 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. III general the section of the wire ought to be chosen with reference to the use which the galvanometer is intended to serve. The 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^[3] 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.^[4] 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.

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

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1. For another definition see the article Electrometer.
2. For A brief history of the construction of galvanometric apparatus see art. Electricity, vol. viii. p. 13.
3. In this and all that follows the silk covering is either neglected or is supposed to vary in thickness as the diameter of the wire.
4. 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 \V. \Veber’s Electrodynamische Jlaasbestirmmmgmz, Thl. ii.; H. XVebcr, Pogg. A)l)l-., 1869; Maxwell’s Electricity and filagnctism, V01. ii. secs. 716 39.11.: Jenkin’s Electrfc'ty and Jlag- netism, cap. xiii. sec. 9.