1888.] the Thermo-electric Properties of Bodies. 231 



covered with clay. The carbon rod was insulated from the platinum 

 wires, and they from each other by thin sheet asbestos and mica, by 

 which means it was insulated from the vessel in which it was placed, 

 and luted with clay to prevent access of air (fig. 10). The numbers 

 obtained in three series are — 





Expt. 1. 





Exnt 2 



Exnt 3 







E. in micro- 











t. 



volts. 



t. 



E. 



t. 



E. 



50 



.... 270 



220 



.... 1800 



210 .... 



1620 



70 



450 



344 



.... 3240 



312 .... 



3024 



88 



.... 540 



499 



.... 5760 



471 .... 



5292 



107 



720 



620 



.... 7560 



635 .... 



8154 



130 



.... 900 



700 



.... 9900 



722 .... 



9990 



160 



.... 1260 











180 



.... 1440 











210 



.... 1620 











The colder junction was at 17° C. 



The resistance of the Pt-C couple was found to vary, increasing 

 to 600°, after which it decreased. This result being caused by the 

 increased resistance of the platinum being partly neutralised by the 

 diminution of the resistance of the carbon, to which must be added 

 the improved contact obtained by the expansion of the platinum in 

 the carbon, which is greater than the expansion of the carbon, thence 

 the pressure increases and the contact improves. 



The numbers were at 220° C. 0'88 ohm, 340° to 500° C. 0"92 ohm, 

 620° C. 1-03, 700° C. 1-00. 



These experiments agree perfectly with the diagram given by 

 Knott and Macgregor (fig. 8) as far as they carried it experimentally. 

 When, however, they commence deducing results for higher tempera- 

 tures, our experiments are not in accord ; there being no indication 

 of the carbon line crossing the platinum line, but only a very slight 

 indication in one of the series of an approach above 230°. 



Assuming the platinum line for our wire to be the same as that 

 given in Tait's diagram (Eleeming Jenkin, p. 178) we get a diagram 

 for carbon (fig. 8a), in which the line is fairly parallel to 250° C, 

 after which it gradually increases its distance. 



Other Changes in the Properties of the Body at the same Temperature. 



This change in the thermo-electric power of carbon is accompanied 

 by other changes. The resistance, the expansion, and the specific 

 heat all appear to undergo a corresponding alteration. 



Resistance. — Accurate measurements of the resistance of carbon 

 at high temperatures are very difficult to obtain, owing to the 

 changes that take place in the connexions. It is desirable, if possible, 



