S71 



GALVANISM. 



GALVANOMETER. 



tuning UM conducting power of metal* for electricity. The follow itu; 

 table shows the conducting power of wire* of the same length and 

 diameter, by which it will be teen that them power* vary with the 

 temperature, the effect of a moderate rue of temperature in reducing 

 the conducting power being considerable : 



SUrer . 



( ,'. . 



Tin 

 Iron . 



Lrad 



Platinum 

 Mercury . 



At It 9 Fahr. 



100-000 



91-517 



64-960 



S4-ST9 



24-06) 



14-014 



13-310 



8-J77 



7-9SJ 



1-738 



Silrtr at 31' Fahr., 

 being equal to 100'. 

 71-318 

 04-919 

 48-48* 

 IT-MI 

 17-S96 



8-657 



8-S87 



0-761 



6-688 



1-279 



At Jl f Fahr. 



100-000 



91-030 



67-991 



J4-M7 



24-073 



12-139 



11-760 



8-078 



8-378 



Ml s 



When equal amounts of electricity traverse similar wires of different 

 metal in equal times the rue of temperature in the wire in inversely 

 proportioned to iU conducting power, so that the better the conductor, 

 the smaller is the quantity of heat emitted by it A compound wire 

 formed of equal alternate portions of silver and platinum will transmit 

 a current so as to heat the platinum links to visible redness, while the 

 silver from its superior conducting power will not become luminous. 



If a Leyden jar or battery be repeatedly discharged through a platinum 

 wire too thick to be fused by it, the wire will become shortened as if 

 it had been acted on by a force transverse to its length. A similar 

 effect takes place with voltaic electricity. If a platinum wire be 

 arranged in a porcelain trough, so that when fused it shall retain its 

 position as a wire, and a strong current be sent through it, it will snap 

 asunder when near the point of fusion, thus showing a contraction in 

 length, while if a similar exj>erimeut be performed with lead wire, it 

 will gather up in nodules which encroach on each other as if from 

 longitudinal compression. It has also been noticed that telegraph 

 wires which have been long in use become brittle, and it has been 

 ascertained that while a wire is transmitting the electric current there 

 is a temporary diminution in the co-efficient of elasticity independent 

 of the heating effect of the current. 



Liquids are greatly inferior to solids in conducting power, so much 

 so that it is difficult to compare them. According to M. Pouillet, a 

 platinum wire conducts 2,600,000 times better than a saturated solution 

 of sulphate of copper. In many liquids the conducting power rapidly 

 increases with the temperature, a result contrary to that obtained with 

 solids. Gases are almost perfect insulators of the voltaic current. 



With respect to the disruptive discharge, we have an instance of it 

 in the fusion and dispersion in vapour of the conducting wire, when 

 not of sufficient capacity to convey the current. By rarefying the air 

 between the ends of the terminal wires, the space through which the 

 discharge will take place may be increased. In the large battery of 

 2000 pairs at the Royal Inxtitutinii, Davy obtained an arc of flame 

 between charcoal points, 4 inches in length. In such cases there is a 

 transfer of solid particles from one pole to the other which increases 

 the effect. A cavity is produced in the piece of charcoal attached to 

 the last platinum or copper plate of the battery, and a mammelated 

 deposit is formed on the charcoal connected with the last zinc plate. 

 Even the densest metals, such as platinum and iridium.are transferred 

 from one pole to the other. 



The discharge by convection introduces us to the beautiful and 

 important branch of science, ELECTRO-CHEMISTRY, to which we refer. 



GALVANISM, in its action on the human system, resembles elec- 

 tricity, yet it is distinguished by certain peculiarities. In its applica- 

 tion it can be rendered more continuous and uniform, and may, like 

 electricity, be administered either hi shocks, or in a regular flow of 

 galvanic influence through the body. It possesses more power over 

 the chemical actions of the body than electricity, and promotes more 

 completely thoae processes of decomposition and rccouiposition which 

 Uke place in the living frame, as well as the functions of organic life, 

 than common electricity. But the chief distinction consists in the 

 difference of action of the two poles. Each pole excites peculiar phe- 

 nomena in the organs to which it is applied. This difference is lew 

 perceptible when mere shocks are administered, than when a con- 

 tinuous stream of galvanic influence is transmitted from one point to 

 another of the body. The positive pole more particularly influence* 

 tli.- iniiw;ul.ir and vascular system, while the negative pole more espe- 

 cially affects the nervous system. At the positive pole there is felt 

 the hock, strong movement*, a feeling of concentration and contrac- 

 tion, increased warmth and mobility of the part, with gradual dimi- 

 nution .f the secretion and sensibility. At the negative pole the pain 

 mod sensibility are stronger and more acute, the organ expands, is more 

 irritable, while the muscular action and mobility are lessened. Thu 

 difference of their action on the secreting powers is best seen by apply- 

 ing the reipcctive pole* to a surface whKh has been recently deprived 

 of its cuticle, such as where a blister has been. The positive pole 

 changes the itcroiu secretion into that of lymph, which at last becomes 

 thready ; the part dries and is inflamed. The negative pole causes an 

 abundant Mention of a dark coloured, highly acrid fluid, which ex- 

 coriates the skin over which it flows; tho part also experience* an 



enduring irritation. Atonic swellings are rendered harder, shoul i 

 not become inflamed by the positive pole, while frequently I 

 negative pole they are dispersed and resolved. Notwithstanding the 

 possession of such powerful properties, galvanism has not produced so 

 valuable results in medicine a* might nave been anticipated. This 

 comparative failure is no doubt to be attributed to errors in the mode 

 of applying it. As tho diseases in which it has been recommended are 

 those already enumerated under electricity [ELECTRICITV, MEDICAL USE* 

 or], it is not necessary to repeat them here. It may be proper 

 ever, to remark, that it was urgently recommended during the preva- 

 lence of the Asiatic cholera, but the results were not satisfactory. 

 Like many other powerful agents, it was not used till a very late stag* 

 in the complaint, when recovery was almost impossible. It -is also to 

 be doubted whether galvanism be at all applicable to cholera, since it 

 appears that the continued application of it causes death, by inducing 

 inflammation of the lungs, in cases of animals where the eighth pair 

 of nerves have been divided, more speedily than where the same nerves 

 have been divided in animals to which the galvanic power was not 

 applied as a substitute for the nervous. Inflammation in the invariable 

 consequence of the application of the positive pole ; while the negative 

 pole would cause a flow of acrid secretion which could not benefit the 

 patient. The identity of electricity, whether common or galvanic, 

 with the nervous power, is much to be questioned, but on this subject 

 Matteucci has entered at some length in his numerous papers. (See 

 Dclarive's 'Electricity;' and the controversy between Dr. W. Philip, 

 Dr. Williams, and others, in ' Medical Gazette,' vol. xvii.) 



The part to which the poles are applied requires to be moistened, 

 where a slight effect is wished, with plain water; but when more 

 powerful effects are wished, with salt and water. 



GALVANO'METER, or MULTIPLIER, is an instrument con- 

 structed for the purpose of detecting the presence of feeble electro- 

 chemical currents. The nerves and muscles of newly killed frogs were 

 at first used ; but the discovery of electro-magnetism has furnished a 

 more delicate and measurable criterion : the instrument founded on 

 this principle has been successively improved in the hands of 

 Schwciggcr, Gumming, Nubili, and Melloni, to a most remarkable 

 degree of delicacy. 



The principle of the construction depends on the property possessed 

 by electrical currents of acting on magnetised needles; for if the 

 conducting wire be placed on the magnetic meridian above or below 

 the needle, the latter will suffer a deviation to the right or k-ft 

 according to the direction of the current [ELECTRO-MAGNETISM.] 



The action of terrestrial magnetism tending to restore the needle, 

 after its derangement by the current, to its original position, is 

 almost entirely corrected by employing two similar needles supported 

 parallel to each other by a light piece of straw or other substance, and 

 placed with the poles of one in an inverse position to those of tho 

 other. A pair of needles thus arranged forms what is termed an aitatic 

 combination, (from aararos, indifferent) The needles are delicately 

 suspended by means of a thread of untwisted bilk or a filament of glass 

 in such a way that the lower needle shall be within a coil of copper 

 wire covered with silk or cotton, and making several hundred convolu- 

 tions. In such an arrangement the needle which is outside the coil 

 will be acted on by the upper wires only, the lower ones not 

 sufficiently near to produce any effect : and this action coincides with 

 their action on the lower needle with its reversed poles, so that tin- 

 effect of a feeble current is materially increased. The apparatus is 

 usually inclosed within a glass case, and there is an adjusting screw at 

 the point of suspension for raising or lowering the sUk, so that wlic M 

 the instrument is not in use the needles can be let down so as not to 

 drag upon the suspending fibre. Over the coil, but below the M|.|..T 

 needle, is a sheet of copper, graduated so as to show the angular 

 deviation of the needles. There are also binding screws for connecting 

 the ends of the coil with the wires which transmit the current. 

 There are also levelling screws and a lever for placing the coil 

 parallel with the needles so as to make them coincide with (In- 

 zero of the graduated circle. Thus the galvanometer not only indi- 

 cates the presence of voltaic action, but estimates its amount. 

 When the needle does not deviate more than 15 or 20 the 

 amount of force may be considered as accurately represented, but for 

 larger angles such is not the case, since the more the needle deviates 

 from parallelism to the wire the more obliquely and consequently less 

 powerfully does the force act. For example, more force is required to 

 move the needle from 20" to 25, than from 10 to 15" ; but as the 

 deviation hi each case is definite for the same instrument the measure- 

 ments may be so far relied on. 



M. Becquerel's IHffertntiai Galvanometer consists of two insulated 

 copper wires of equal length and diameter surrounding the needles, 

 forming two independent circuits, and having four terminations instead 

 of two. This instrument was used for estimating the conducting 

 power of the different metals for electricity which varies nearly in the 

 same order as their power of conducting heat Now it is evident that 

 if two equal currents were sent in opposite directions through the two 

 coils of the above instrument they would neutralise each other in 

 their action on the needle which would remain undeflectod ; but if one 

 current were stronger than the other there would bo a corresponding 

 deviation of the needle. In using the instrument a small voltaic 

 battery was connected with it, witli two wires from each pole so as to 



