GALVANISM. 



obtained, is the mechanical energy required to be 

 expended. 



Several modifications of Wilde's principle have 

 been introduced within the last few years by 

 Siemens, Wheatstone, Ladd, and others. Ladd's 

 improvement makes the machine more convenient 

 and simple in form, and promises to make it more 

 popular. Theoretically, these machines are ex- 

 amples of the mutual convertibility of heat or 

 mechanical energy into kinetic electric energy, 

 and of this again into the energy of mechanical 

 force or of heat and light. 



THERMO-ELECTRICITY. 



As the electric current produces heat, so we 

 may invert the order of the phenomena, and from 

 heat derive electricity. When any two metals, 

 unequally susceptible to heat, are soldered to- 

 gether, and heated at the joining, an 

 electric current is evoked. The two 

 metals which shew this property most 

 -N. readily are antimony and bismuth. Thus, 

 A if a bar of antimony, A (fig. 43), be 

 f soldered to a bar of bismuth, B, and 

 their other ends connected with a gal- 

 vanometer, G, a current will pass when 

 we heat the junction, S. Inside the 

 couple, it flows from bismuth to anti- 

 mony, and outside from antimony to 

 bismuth, as we know by the side to 

 which the needle swings. If any two of 

 the following metals be joined in this 

 HEAT way as a thermo-electric pair, a current 

 Fig. 43. will flow, at the heated junction, from the 

 one which occurs first in the list to the 

 other bismuth, nickel, lead, tin, copper, platinum, 

 silver, zinc, iron, antimony, tellurium. 



If cold, instead of heat, be applied at the junc- 

 tion of two metals, it will produce a current oppo- 

 site in direction to what the heat would. When 

 we plunge the antimony-bismuth couple in ice- 

 water, a current flows from bismuth to antimony 

 in the outside wire. One antimony-bismuth pair 

 has little power, but several may be joined, just as 

 in a galvanic battery. Fig. 44 shews the arrange- 

 ment of such a thermo-electric 

 pile or battery. Obviously, only 

 the odd junctions must be 

 heated, for if all were, there 

 would be no current at all. 

 The current is from bismuth to 

 antimony when a junction is 

 heated, and from antimony to 

 bismuth when it is chilled. 

 Thus the strength of current 

 produced will depend on the 

 difference of temperatures in 

 the two sets of faces. A pretty 

 strong current may be evolved 

 by keeping one set of junctions 

 in ice, or a freezing mixture, and the other in 

 boiling water. The electro-motive force is, how- 

 ever, very weak compared with that due to chemi- 

 cal action. In fact, it takes about one hundred and 

 fifty bismuth-antimony pairs to give the electro- 

 motive force of a single Daniell cell, one set of 

 joints being kept at the freezing, and the other at 

 the boiling point of water. For this reason, heat 

 is not available as a convenient source of elec- 

 tricity by any of the means yet discovered. 



C COLD \ 



L ^ 



HEAT 

 Fig. 44. 



It is as a thermometer that the thermo-electric 

 battery has its most important application. 

 When employed for this purpose, it is called a 

 thermo-pile. To form it, some twenty or thirty 

 antimony-bismuth pairs are connected as in a 

 battery, and compactly put together with var- 

 nished paper between the sides. The whole is 

 inclosed in an insulating tube, so that only the 

 junctions are exposed at each end, and these are 

 blackened, to increase their absorption of heat. 

 Wires connect the end plates with an astatic 

 galvanometer, whose coil must be of short thick 

 wire, on account of the feeble tension of heat- 

 excited currents. If either face be heated, a 

 current will arise : if one be heated, and the other 

 cooled, the current will be stronger ; but if both 

 be heated alike, there will be no current. So 

 sensitive is such an instrument, that the mere 

 approach of the hand or of a piece of ice, though 

 at a considerable distance, is sufficient to deflect 

 its galvanometer needle. For experiments on 

 radiant heat, this instrument is most important, 

 and now supersedes all others. 



A curious effect was observed in 1834 by Peltier, 

 and is sometimes known as the Peltier effect. 

 When a feeble galvanic current is sent through 

 an antimony-bismuth pair, in a direction from 

 bismuth to antimony, and the junction is im- 

 mersed in water, cooled down to the freezing- 

 point, but not frozen, as much cold will be devel- 

 oped as will make the water freeze. Heat will 

 be produced if we reverse the current, and the 

 water will not then be frozen. In a series of pairs 

 the junctions will be alternately heated and chilled. 

 Only the current must not be too strong, else the 

 whole will be heated. 



All these facts have been shewn to be in com- 

 plete accordance with the modern doctrine of the 

 conservation of energy. If the two faces of a 

 thermo-pile be kept each at a constant tempera- 

 ture, a constant current flows, in the antimony 

 plates, from the warmer junctions to the other ; 

 and the energy of the current is an exact equiva- 

 lent of the excess of heat-energy absorbed at the 

 warmer over that given out at the colder junctions. 

 If the current be not converted into magnetical, 

 mechanical, or chemical force, the heat energy 

 will be entirely recovered as heat in the circuit. 

 But if the current be used to do any work, a cor- 

 responding amount of heat will be withdrawn from 

 the circuit. 



ANIMAL ELECTRICITY. 



The tissues and organs of animals are not only 

 affected by the electric spark and current, but 

 they are themselves also a source of electricity. 

 In general, the strength of their electro-motive 

 power is so feeble that it requires special experi- 

 ment to detect it. There are, however, some 

 species of fishes which exhibit such a power very 

 readily. 



It was long ago known that a certain kind of 

 flat fish, somewhat like a skate, and found on the 

 shores of the Mediterranean, produced a numbing 

 sensation in the arms of the fishermen when they 

 handled it. The Romans called it torpedo, from 

 a word meaning to ' benumb,' and this name is 

 now usually given to the whole genus to which 

 it belongs. It has regular electric organs or 

 batteries, placed on each side in the spaces 



