GALVANISM. 



GALVANISM. 



exposing but a very small surface, and excited with distilled water 

 only. Such an arrangement is called a water battery, and when well 

 insulated its effects are remarkable. With 1000 couples divergence of 

 the guld leaves of an electroscope was produced, a charge was given to 

 Leyden battery, which, though weak, could be discharged and renewed 

 any number of times in rapid succession. In Mr. Gsniot's arrange- 

 ment, when the wires terminated in brass disks, placed very near 

 together, a rapid succession of sparks was kept up, on one occasion, for 

 five weeks; the galvanometer was ; permanently deflected, and paper 

 moistened with iodide of potassium gave indications of decomposition, 

 although the chemical effects of the water battery are but feeble. 



Under ELECTRODYNAMICS will be found some formula) for esti- 

 mating the force and resistance of galvanic currents ; and we gave a 

 brief statement of Ohm's method of representing the mutual actions of 

 the electro-motive forces, and the resistances of any circuit in the form 

 of a fraction. Now as the power of any combination, simple or com- ; 

 pound, is directly proportioned to the electro-motive force, or chemical 

 energy between the active metal, and one of the elements of the liquid 

 on which it acts, and inversely proportioned to the resistances to be 

 overcome, the numerator of the fraction will be represented by E, the 

 electro-motive force, and the denominator by R + r, R representing the 

 resistance in the cell or the battery (chiefly due to the affinity between 

 the elements of the liquid for each other), and r all resistances exterior 

 to the cell and the battery, such as that of the connecting wire : then the 



expression 5- = A would represent the effect of any combination where 



A indicates the amount of force in circulation, whether measured by its 

 heating or by its magnetic effects. If the connecting wire be very thick, 

 so as to offer little or no resistance to the current, r goes out, and the 



fraction becomes ? = A. Now, suppose that when a pair of zinc and plati- 

 num plates, 6 inches in length and 1 inch in breadth, and 1 inch apart, is 



immersed in dilute acid, E = l and R = l then f=-=l. If a pair of 



B 1 



plates, 6 inches broad and 6 inches long, be immersed in tho same acid, 

 since the resistance is inversely as the surface of the plates immersed, 



E 1 



the fraction becomes B or 1 = 6, or the power is increased six-fold, as 



6 6 



compared with the former. If each of the plates be cut into six 

 similar slips, and these be arranged in pairs, all the platinum plates 

 connected by the same wire and opposed to all the zinc plates simi- 

 larly connected, the same fraction still represents the result, since the 

 relative size and distance of the plates remain unchanged ; but if the 

 plates be made to alternate, the zinc of one pair connected by a wire to 

 the platinum of the next pair, so as to produce a compound circuit, 



the fraction becomes _!! = - = 1, the electro-motive force is increased 



lil! 6 



nix-fold, but the resistance is also increased in the same proportion. 

 The force, which in this case circulates through the connecting wire, is 

 not greater than if a single cell, containing a pair of plates 1 inch broad 

 and 6 inches long, had been employed. If, however, the connecting 

 wires were several miles in length, as is the case in the electric tele- 

 graph, r becomes important. We will suppose tho resistance to be 

 twenty-fold greater than that of the liquid in each cell : then, in the case 



of the simple circuit, the fraction becomes K 



jj + r 



+ 20 = 0-049; 



and in the case of the compound circuit, 



6E 



6 



_ 

 20 



= 0-23; so that 



although in both cases the resistance greatly diminishes the amount of 

 circulating force, the power in the compound circuit is five times as 

 great as that produced by the simple circuit, so that wherever there is 

 a great external resistance, a compound battery is very much to be 

 preferred to the eimple circuit. For, let = the number of plates in 

 n compound circuit, E the electro-motive force, D the distance between 

 the plates, s the area of the plates, / the length of the conducting wire, 

 the area of a section of the wire, then the action of a compound 

 battery with its extremities connected by a thick metallic wire, is 



BE 



expressed by the fraction np |= A. In Professor Miller's ' Elements 



8 + 



of Chemistry,' part i., the reader will find some further instructive 

 applications of Ohm's theory. The chemical decompositions produced 

 by the galvanic battery are stated under ELKCTHO-CHEMISTRY ; and 

 the brilliant light furnished by the voltaic are is noticed under 



n tc LIOHT. See also ELECTRICAL I 



GALVANISM. This department of electricity takes its name from 

 Galvani ; but its infant progress was due in a much greater degree 

 to his contemporary Volta, by whom piles were first constructed for 

 increasing the intensity of the electricity produced by a single pair of 

 platem. The production of electricity in this case arisen from the 

 action of the acid in the cell between two plates of dissimilar metals, 

 that which is the more oxidable giving out positive electricity, as 

 explained under ELF.CTIIO-DTSAMICS. The forms in which the piles 



have been constructed are various, and the number of plates is n. i 

 either to the quantity or intensity of electricity which may be desired. 

 When quantity with a feeble tension is requisite, a single pair of plates, 

 such as cine and copper, with extensive surfaces, separated by very 

 dilute acid, will answer; but with a system of pairs of plates, where 

 the copper of the first pair rondurtt its electricity to the lino of the 

 second, and so on, the quantity and intensity are increased with the 

 number of the plates. In some constructions, as Hitter's dry piles, 

 the plates are 'simply laid on each other, those of each pair being 

 separated by moistened paper ; in others the plates lie parallel in a 

 trough of baked wood, by which means the cells are easily filled and 

 emptied. In the Couronne de Tatta of Volta the plates are placed 

 circularly or in a bowl shape ; while in Hare's Cahrimut'ir there is 

 merely one zinc plate and one copper twisted into a great number of 

 coils, which fonn increases the intensity, as may be seen from the 

 article ELECTRO-DYNAMICS. All these forms have however been 

 superseded by tho constant battery, as noticed under GALVANIC 

 BATTERY. 



The electricity thus produced is of the same nature as that given by 

 the common machine; the only difference being that the mode of 

 producing galvanism is continuous, that is, when in any way discharged 

 it is immediately reproduced by the oxidation of the zinc ; and hence 

 many galvanic phenomena have been successfully imitated by a series 

 of sparks of ordinary electricity. When the positive and negative 

 wires are made strictly to communicate by metallic conductors, the 

 combination of the opposite electricities causes all phenomena analogous 

 to those produced by the ordinary machine to cease, but gives birth to 

 the electro-dynamic and electro-magnetic phenomena. [ELECTRO- 

 MAGNETISM.] But when the wires from the opposite poles of the 

 battery are only brought sufficiently near that the current may pass 

 through an interposed substance, or when the circuit is completed by 

 imperfect conductors, the physical changes which the interposed 

 substances undergo constitute the phenomena of galvanism. It may 

 be observed that the relative conductibility of substances for Voltaic is 

 nearly the same as for common electricity, but the alterations pro- 

 duced by the former in the temperature and internal nature of the 

 substances through which the current is admitted interfere in some 

 degree with that order of conductibility. 



The deflagration of metals is effected by beating them into thin 

 leaves, which are then interposed between the extremities of tin- 

 positive and negative wires of the battery, brought within a quarter of 

 an inch of each other : they will then burn with a beautiful light, but 

 which is of different colours in different metals. Thus zine gives a 

 white light with a reddish border ; copper, a bluish white light, and 

 throws out red sparks; lead, a purple light; gold leaf, a beautiful 

 white light tinged with blue. 



But if the interposed substances, instead of being lamina;, be of 

 small irregular forms, or wire-shaped, their temperature rises rapidly 

 as the electric current permeates them. Steel bums, iron wire 

 dissolves in globules, while charcoal produces a light of such dazzling 

 brilliancy as to fatigue the eye, a property which has been happily 

 seized by employing it in the solar microscope [ELECTRIC LIGHT] ; yet 

 this heat and light arc independent of the ambient medium, no oxygen 

 is consumed, and the attenuation of the air rather odds to than 

 diminishes the light. As for the apparent diminution of this 

 light when the charcoal is immersed in water, it is attributable to 

 the imperfect conductibility of the latter medium; a thermometer 

 placed in water, in which tho wires are immersed, will rise even to the 

 boiling point. Mr. Children has given a list of the order of facility in 

 which substances thus acquire a red heat, and has succeeded in i 

 the oxides of molybdenum, tungsten, uranium, &c., but found ruby, 

 sapphire, silex, quartz, &c., more intractable. It is obvious that, in the 

 estimation of such an order, we must take an account of the mass 

 heated, and of the extent of its surface which is liable to cool by 

 contact, radiation, or both ; and lastly, of the loss of conductibility duo 

 to the increase of temperature of the substance interposed. Ktln-r, 

 alcohol, &c., may be inflamed, and gunpowder exploded, by making the 

 dii- charge through charcoal points. 



Sir Humphry Davy avoided the increase of temperature in the 

 through which the current was discharged by taking them of a li n tit 

 sufficient to discharge the number of pairs of plates employed in tin- 

 pile, and thus found that the length of wire in this case is in 

 proportional to the number of double plates. The diminution of 

 conductibility due to increase of temperature he exhibited l>y a 

 platinum wire made red-hot by the galvanic current; for when In- 

 raised one part of it to a white heat by means of a blow-pipe, 1 1 

 in the other parts of the wire became immediately ndinvd. Tho 

 order of heating in metals, beginning from that most susci ; 

 which he bos given, is as follows: iron, palladium, platinum, tin, 

 zinc, gold, copper, silver. 



Under ELECTRO-CHEMISTRY will be found a statement of 

 subsisting between < In niical changes an rieity. We 



may however in this place state some of the early results obtained. 



The decomposition oi water by tin- K <'ted by bringing 



the points of the positive and negative wires very near each other 

 under water, inverted glasses being placed over them to collect the 

 gases which ore evolved. If the wires be not oxidable, then o 

 gas will bo formed at tho extremity of the positive wire, and hydrogen 



