PAIO KLECTRICITT. 





VOLTAIC ELECTRICITY. TUT. 



HOD KB or OKNEKATINO KLKCTKICITY BY HEAT THEKMO 

 . ILK Kg EFFECT OF CUUIIKNTH ON MAONET1HKIJ 



NEEDLE UAI.VANOMETEU HEFUCCTINtJ UALVANOMKTKK 



TANUKNT COMPASH CHEMICAL EFFECTS. 



r* have now to ascertain the mode in which an electric current 

 be produced by heat. It wa discovered by Professor 

 ok, in 1822, taut if two ban composed of different metaU, 

 even of the same metal in different conditions u 



, <M*tiin.l Immmored brass be joined together, and heat 

 ;.plii-.l t<. tin- point of junction, an electric current will be pro- 

 Tht- bust plan of trying this is to solder them together, 

 !i.J tlifn I., IK! the strip into the shape of the letter U, as shown 

 Fig. 44. Wires from the free ends are now connected with 



of the arrangement shown in Fig. 45. A plate of bismuth, op 

 has a strip of copper bent down at the end*, and soldered to it 

 A compass-needlo, a, is balanced on a point between the two, ana 

 the whole apparatus is supported on a stand, and placed so that 

 tho sides of the metal bars are exactly parallel to the FTffflt 

 Now let a spirit-lamp bo applied to the junction at one end, and 

 the needle will be at once deflected By obaerring the way in 

 which the needle points, we shall find that the current pinst 

 along tho copper in the direction shown by the arrow, and back 

 along the bismuth from p to o ; that is, it travels along the bis- 

 muth from the colder to the warmer end. 



By placing a lump of ice on the end m at the same time that 

 the lamp is applied to n, the deflection of the needle will be in. 

 creased. If, on the other hand, heat be applied to both 

 the effect will be diminished ; and when both are at tho 



the screws of the galvanometer, o, and wo shall find that if the 

 flame of a spirit-lamp, or any other source of heat, be applied 

 to the point oi junction, c, the needle will be deflected. If 

 we now remove the source of heat, and drop a little ether on c, 

 or lay a lump of ice on it so as to reduce the temperature 

 there below that of the rest of the bar, the needle will be 

 deflected the other way, showing that now the current is passing 

 in the contrary direction. 



It is not even necessary to have two different metals to 

 exhibit these effects. If wo take two pieces of the same copper 

 wiro, and, having connected one with each polo of the galvano- j 

 meter, heat one to redness and let it touch the other, a faint 

 current will at once pass from the hot to the cold wire. These 

 currents are called therm.o-eJect.ric, to distinguish them from 

 hydro-electric currents, or those produced by means of liquids. 

 They possess but little intensity, and therefore galvanometers 

 for use with them have to be specially constructed, thfl wire 

 being short and thick, so as to offer as little resistance as 

 possible. 



A more elegant plan for exhibiting these effects is by meann 



125 K.R. 



temperature, the needle will remain in the same position that it 

 occupied at first. We see, then, that it is not the degree of 

 temperature which any part of the metal has, but the difference 

 between the temperatures of its two extremities, that gives rise 

 to the current. 



The two metals employed in this experiment are bismuth and 

 copper. Had a piece of antimony been employed instead of the 

 copper, the current would have bdan more powerful. By trying 

 the comparative effects thus prod($ed by a number of different 

 metals, we are able to arrange tti&ra in a series, jnst as we did 

 when considering their employment in ordinary batteries 

 (Lesson II.). 



As a result of these experiments, tho metals are arranged in 

 the following order : 



Bnmutli. Aluminium. Copper. Iron. 



Nickel. I'-- -. Platinum. Antimony. 



Gorman SUrer. Lead. Silrer. Tellurium. 



Mercury. Tin. Zinc. 



If a bar composed of any two of these be heated at the junc- 

 tion, the current will pass at that point from ths one that u 



