KNOBLAUCH ON RADIANT HEAT. 189 



length, and the opposite side with a cyHnder of the same 

 diameter, but only 19 miUim. in length. Both by these and by 

 screens properly placed, the pile is protected from all extraneous 

 rays, so that, excepting the temperature of the surrounding air, 

 which acts equally on all sides, it is only exposed to the influence 

 of the source of heat. 



The conducting wires which connect it with the multiplier 

 are fastened, by means of binding screws, to copper sockets, in 

 which the poles of the pile terminate. 



The Multiplier is constructed on Nobili's principle, which ap- 

 peared to correspond to the desired object better than any other 

 recently proposed arrangements, and its accuracy has been com- 

 pletely verified in the series of experiments about to be detailed. 



One advantage in my instrument consists in the wire being 

 composed of electrotype copper ; hence the disturbing influence 

 of the iron* contained in the ordinary conducting wire was 

 avoided. Certainly the wire was drawn through steel, instead 

 of which a ruby might have been used; however, there is no 

 fear of the wire becoming contaminated with iron from this 

 source, because the aperture through which it passed was com- 

 pletely covered with copper, and moreover it was carefully 

 virashed in dilute acid before being covered with the silk. How 

 fiiUy the object was attained in this way is evident, among other 

 circumstances, from the double needle with the purified copper 

 Ayire remaining only 1°'5 on either side of zero on the scale; 

 whilst in the coils of an ordinary conducting wire it could not 

 be approximated more than to 20°. The slight deviation of 1°'5, 

 which arose from the magnetism of the copperfj might certainly 

 have been avoided by closing the fissure between the coils J; 

 but other circumstances appeared to render this unadvisable. 

 The length of the copper wire which surrounds one needle of the 

 astatic arrangement in 160 turns, is 31-5 millim. ; its thickness is 

 I'l miUim. The mean length of the coils of wire is 9 centim. 

 2*5 millim. ; their breadth, 4 centim. 6*5 millim. ; their mean 

 distance from each other, 15*0 millim. The two equal portions 

 of which the galvanometer wire consists, and which being wound 

 around each other form the coils, may be combined so as either 

 to allow the current to pass through them successively, and thus 

 singly the whole length of the wire, or so that it simultaneously 



• Moser, Dove's Reperl. vol. i ]>. 261. 



t II. Schroder, I'ogp;. Annal. vol. liv. p. 59, and vol. Ivi. p. ."339. 



X Pcclct, Ann. de CIdm. et tie Phyn., ser. 3, vol. ii. p. lO.J, 



