30 



On the Thermal Conductivity of Vulcanite. 



Experiments (d) to (o) . — By these standard disks the con- 

 ductivities of 12 other specimens were easily obtained. I 

 give in the next Table the results, not in the order in which 

 the experiments were performed, but, for convenience, in the 

 order of the conductivities of the specimens. 



Specimens. 



Thickness in 

 centims. 



Temperature L, -, .... 

 Limits. j Conductivity. 



D 



1-584 44-4 and 16-5 



0000317 

 0-000313 

 0-000309 

 000307 

 0000305 

 0000282 

 0-000256 

 0-000254 

 0-000237 

 0000217 

 000214 

 0-000200 



E 



F 



G 



1-265 ' 40-1 „ 15 8 

 1-297 40-1 „ 149 

 1-254 ! 40-5 „ 16-1 



H 



I 



J 



K 



L 



0-979 370 „ 16-4 

 0-499 500 „ 23-9 

 1-003 370 , 121 

 0-945 36-1 ., 12-6 

 0-797 \ 35-2 „ 12-5 

 0-643 | 33-1 „ 13 2 

 0-644 ' 32-8 .. 122 



M 



N 







0-498 



30-5 „ 12-8 



In view of the suspicious decrease of the thermal conduc- 

 tivity with the thickness of the piece experimented upon, it is 

 well to say that some manufacturers apparently use rubbers 

 of different compositions for thick and thin plates. Much 

 cheap, thin rubber is to be had in the market, and the specific 

 electrical resistance of some of this is so low as to make the 

 material useless for insulating purposes. Of the specimens 

 mentioned in the table, Gr and H are pieces of expensive 

 rubber made especially for use in induction-coils of high 

 grade. K, which is nearly of the same thickness as H, is an 

 excellent rubber of high electrical resistance, but is not quite 

 equal in this respect to H, which has a very different thermal 

 conductivity. Extended experimentation seemed to show 

 that the indications of the thermal elements might be trusted 

 whether the plates were thick or thin. A re-determination of 

 the conductivities of N and O with the different arrangements 

 of the prisms gave 0-000215 and 0-000199 respectively ^ When 

 a disk of the standard rubber was turned down thin and its 

 specific conductivity re-determined, it was found to be almost 

 the same as before ; but when disks F and I were made slightly 

 thinner by turning off the polished surface on each side, the 

 conductivity of the thinner disks was increased by 5 per cent. 



