46 



Mr. H. H. Poole on the Thermal Conductivity 



Then if Q calories be generated per second per centimetre 

 length along the axis and A he the difference of temperature 

 between two points distant/'! cm. and r 2 cm. respectively from 

 the axis, we have 



K= A-log.3, 



2ttA ri 



where K is the thermal conductivity. This equation of 

 course only holds for points sufficiently remote from the 

 ends of the cylinder and for steady temperatures. 



The rock is placed in a tube electric furnace maintained 

 at a constant temperature ; along the axis of the cylinder a 

 hole is drilled through which runs a platinum wire by means 

 of which heat can be electrically generated at a known rate. 

 The temperatures are measured by means of platinum, 

 platinum-iridium thermo-couples. As the boring of fine 

 holes especially in the harder rocks is exceedingly difficult, 

 some other method has to be adopted for sinking the couple- 



in the rock. The cylinder is cut in two by a central plane 

 perpendicular to the axis. The two halves arc ground 

 together with fine carborundum powder until an accurate tit 



is attained. A system of tine grooves :i- shown in tig. 1, 



