114 Dr. 0. J. Lodge on a Method of measuring the 



(2) When the crystal and rods are covered over with a coat 

 of varnish (as Brunswick black), so that h is the same for all. 

 In this case the coefficient of the right-hand member becomes 



simply ~2j but the treatment required is the same as in the first 



case. 



(3) When the crystal is surrounded with cotton-wool or felt, 

 or in some other way has its exterior surface made adiabatic, 

 so that h' = Q. In this case the left-hand member of the above 

 equation equals z, and the right-hand becomes indeterminate, 

 so that a fresh investigation is necessary. 



Case ivhen radiation from the exterior surface of the cylinder 

 is prevented. 



5. Heat will now flow through the cylinder as through part 



dO . © / — © 



of an infinite wall, and -j- becomes simply ; hence the 



two sets of equations (1) and (2) are now all equal to one 

 another, and they reduce to 



7 dT ©-T ®'-0 T / -@ / ,„. 



k-r-=K —X =« • • • (<) 



dx y * z y 



But as these are only three equations between four unknown 

 quantities, some further observation is necessary before we can 

 determine %. We may either omit the crystal altogether, or, 

 what is probably better, replace it by a piece of the same metal 

 as the rods are made of, and repeat the temperature-determi- 

 nations, the packing being kept exactly the same as before. 

 Denoting the temperatures in this case by small letters, the 

 equations will now be 



*£«(^)-=^-«(^). ■ ■ « 



where z f is the thickness of the bit of metal. By these three 

 equations - is determined ; and its value may then be substi- 

 tuted in the former set. 



Eliminating © and W from the former set (7), we have 



(?+¥)£-" <»> 



similarly from (8) we get 



'z^ + M^^t'-t 

 , k k ) dx 



