GALVANISM. 



GALVANISM. 



270 



determined to the conducting plate instead of the hydrogen, which 

 plate therefore remains unacted on ; in the other portion of fluid the 

 wire connected with the copper plate becomes the analogue of the 

 zinc, and the water undergoing decomposition at its surface, the 

 oxygen unites with the copper of the wire to form a protoxide, which 

 combines with the free sulphuric acid, while the copper of the 

 sulphate is again determined in the direction of the current, in pre- 

 ference to the hydrogen, to the surface of the other wire. 



This example explains why, during electrolytic decomposition, the 

 elements of a compound are determined in definite directions to one 

 or other pole of the battery : and hence the classification of those 

 elements into electro-potitive or electro-negative, or into Cations and 

 , according to the nomenclature proposed by Professor Faraday. 

 [ELECTRO-CHEMISTRY.] 



When the elements of a voltaic arrangement are single, or when it 

 consists of a pair of plates only, the current, however abundant as 

 regards what may be called quantity, is deficient in tension, or in that 

 quality which pre-eminently characterises the electricity of friction : an 

 increase of tension, as well as of quantity, is obtained by combining two 

 or more single batteries, the copper element of the one being connected 

 with the zinc of the next by a perfect conductor, and so on in continued 

 succession. In constructing such compound arrangements, or batteries, 

 it must not be inferred that the quantity and tension depend simply 

 on the extent of the surface of the plates taken collectively, and on 

 their number ; it has been proved by Daniell, Faraday, and others, that 

 the maximum effect ia obtained from a given amount of surface and a 

 given number of combinations, when all the plates are exactly equal ; 

 and that if one pair of the series is much larger or smaller than the 

 rest, there will be no additional power gained in the first case, and 

 a considerable loss in the second, nearly equal to what would have 

 been produced if all the plates had been reduced to the size of the 

 smaller. 



The term tension, as applied to the property imparted to the voltaic 

 current by compound combinations, is, perhaps an injudicious one ; 

 since it suggests the idea of some resemblance to the quality of 

 frictional electricity alluded to, which, as will be presently stated, is 

 not precisely the case, but that a very decided modification of the 

 active qualities of the current is produced by these combinations is 

 shown by the fact that the decomposition of a fluid electrolyte into its 

 constituent elements cannot be effected by a single pair of plates, 

 however extensive their surface; while this decomposition becomes 

 energetic by means of a combination of four or five small batteries, 

 the total surface of the plates of which is far less than that of the 

 single one. 



The most energetic form of small constant battery is that suggested 

 by Mr. Grove, the elements of which are amalgamated zinc and sheet 

 platinum, the latter being immersed in concentrated nitrous acid, 

 while the zinc element is plunged in dilute hydrochloric acid isolated 

 by porous chambers. The platinum is not acted on, and consequently 

 the plates are lasting, although costly at first. The zinc is of course 

 rapidly destroyed, as it must be in all energetic arrangements, for 

 reasons before given. 



In constructing the science of electricity, the mind has, as in other 

 sciences, proceeded from first observed facts to simple generalisations, 

 or theory, which have again served as guides to new observations and 

 more comprehensive generalisations. Small as the progress yet made 

 has been, and slight as our knowledge is, when compared with what 

 remains to be acquired, we are warranted in concluding, from the 

 steady adherence to the principles of inductive philosophy which has 

 long characterised our investigations, that the present theory of this 

 science will be an admitted one, divested as it is, and ought to be, of 

 any attempt to explain ultimate causes, and limited to comprising in 

 general expressions the combined results of past observations, to be 

 tested by its accordance with new ones as they occur. What electri- 

 city is, or more correctly speaking, what is the nature of that unknown 

 agent which is the cause of electrical action in the most comprehensive 

 sense of this term, we are utterly, and probably ever shall be, ignorant ; 

 but this agent appears to be as inseparable from matter as gravitation, 

 and polarity appears to be the consequence of its presence in an active 

 state. It can be elicited, or brought into this active state, by different 

 causes, and what is remarkable, it presents some striking modifications 

 in its sensible properties, according to the cause which has immediately 

 called it into action ; it was these modifications which prevented iia 

 recognition when first obtained by chemical action, or by other than 

 mechanical means, and caused it to be denominated galvanitm or 

 ttUaitm, and yet longer concealed the constant connection, if not 

 identrty, between it and magnetism. 



That the ultimate cause of these modified modes of action is 

 identical, we are warranted in inferring from the identity of the phe- 

 nomena which they all produce, and from the interchangeable relation 

 of cause and effect that exists between these various sources of elec- 

 trical excitation ; thus we infer that the unknown agent alluded to is 

 the cause of electricity, voltaism, magnetism, and heat, for each of 

 these may be produced, and each in its turn is capable of producing 

 the others. Friction is the source of an electrical action which 

 prodoon light, heat, chemical action, and magnetism, but chemical 

 action is the most abundant source of this agent, and all these pheno- 

 mena are produced more freely by it than by the others ; nevertheless 



the presence, even in the least appreciable degree, of any one of them, 

 warrants us in admitting the identity of the cause. Magnetism is 

 always co-existent with the agent alluded to, but under certain con- 

 ditions of direction of the electrical and magnetic polarities. Whether 

 that agent be elicited by mechanical or chemical action, or by changes of 

 temperature in different metals [THERMO-ELECTRICITY], and conversely 

 by employing magnetism as the immediate exciting cause, chemical 

 action, heat, light, and magnetism itself may be produced. [MAGNETO- 

 ELECTRICITY.] 



The most remarkable and obvious of the modifications which have 

 been above alluded to are those which have given rise to the terms 

 electricity of tension, and current-electricity, the former characterising 

 frictional or mechanical, the latter chemico-, thermo-, and magneto- 

 electricity ; but it is now acknowledged that these terms are only 

 expressive of the highest and lowest degrees of a common property. 

 The spontaneous disruptive discharge which takes place through very 

 imperfect conducting media, such as dry gases, from a body supercharged 

 with frictional electricity, has never yet been produced by the most 

 redundant quantity of chemico-electricity. Contact must be made by 

 some good conductor before the current can pass ; although the current 

 will continue for a short time after that contact is again broken, but 

 only through a small intervening space, never exceeding an inch or two, 

 between the anode and the cathode, the continuity of the current under 

 these circumstances being indicated by the intense arc of flame between 

 the points terminating the connecting wires of an extensive compound 

 battery ; and in the case of thermo- or magneto-electricity it is only at 

 the instant of breaking the contact that the spark appears, indicating 

 the momentary transit through an unappreciable distance of the current, 

 which is instantly arrested when that distance becomes sensible : yet 

 that the electricity from these three sources possesses some tension 

 has been proved by the charge imparted to a Leyden jar from a voltaic 

 battery, and by other indications of the presence of that quality in 

 feeble degrees, both in thermo- and magneto-electrical currents, as for 

 example by the decomposition of solution of iodide of potassium when 

 the anode and cathode are very near to each other. 



That power to which the name of induction is given, and which is 

 the consequence of the more comprehensive agency termed polarity, is 

 another and universal characteristic of electrical action. [ELECTRICITY, 

 COMMON.] The inductive power of frictional electricity and of mag- 

 netism were the earliest observed facts in the science, but it is only 

 within a few years that the same power has been shown to accompany 

 the other sources of excitation, owing to the low degree in which they 

 possess it when compared with the former : but now by means of the 

 multiplier or coil, the inductive power of the current from a single 

 pair of plates may be made manifest, especially by the physiological 

 action called the electrical shock, which thus modified cannot be dis- 

 tinguished from that obtained from a charged Leyden jar. 



We must also glance at another source of electricity namely vitality. 

 Are wo yet warranted in assigning this as another ett'ect of the same 

 agent ? This question cannot be answered ; all we as yet know is, that 

 animal electricity as exerted at volition by the Gymnotus, Torpedo, 

 and perhaps other animals, is capable of producing induction, and 

 therefore attraction, heat, light, chemical action, and magnetism, and 

 the physiological action on living bodies, identically in the same manner 

 as the electricity from inorganic sources. [ELECTRICITY OF ORGANIC 

 BEINGS, in NAT. HIST. Div.J 



In comparing the discharge of the galvanic battery with that of the 

 common electrical machine, we may remark that the current is a con- 

 tinuous succession of discharges of electricity generated and maintained 

 by the contact and chemical action of the materials of the battery, and 

 like the discharge of the electrical machine it may be classed under 

 three heads, namely, the discharge by conduction, as when the circuit 

 is completed by a good solid conductor ; by disruption, when a luminous 

 appearance is seen through a short interval of non-conducting matter ; 

 and thirdly by convection, which takes place in liquids, and is accom- 

 panied by chemical action, and the transfer of particles of the 

 conductor. 



When electricity, however produced, is in motion, all the particles 

 of the conductor are concerned, in conveying the force, and not the 

 surface merely, as in static electricity. [ELECTRICITY.] In such a case 

 induction takes place between one transverse section of the conductor 

 and other sections just before and behind it, while a small but sensible 

 portion of the induction is directed to surrounding objects. If the 

 thickness of the conductor be reduced the particles have to transmit a 

 larger portion of the force, and the result is a considerable elevation in 

 temperature, sufficient to ignite and even fuse the wire, a circumstance 

 taken advantage of in firing the charge of gunpowder used in blasting. 

 Metals degenerate in conducting power by elevation of temperature, as 

 may be well shown by raising a platinum wire to a dull red heat, 

 1 by sending a voltaic current through it, and heating a portion of the 

 wire by means of the flame of a spirit-lamp; the temperature of the 

 other part of the wire will decline, and the wire cease to be visible, in 

 consequence of the increased resistance offered to the current by that 

 part of the wire which is in the flame. A contrary effect is produced 

 by cooling a portion of the wire in water, the reduced temperature at 

 1 one point allowing more electricity to pass, as is manifest by the light 

 given off by the wire, which may even fuse and disappear in globules. 



We have stated under GALVANOMETEK the method adopted for ascer- 



