OP CRYSTALS AND OTHER BAD CONDUCTORS. 
493 
Column (5) gives the values of (l + a v-~j — j , and column (6) gives the 
i/,r 
values of 
—— Jv\z (dv„y 
1 +a " f fa) "life) 
v % ^ 1 4* 2 —v + V 
which by equation (6) = ph/qk. 
Neglecting the first two numbers in column (6), which are uncertain on account 
of the smallness of the quantity (5) at the cool end of the bar, we have, as the 
mean value of ph/qk, ‘00141. 
Hence 
ph 
k = 
•00141 q 
2'093 x ‘0001804 _ ‘0003777 _ p gram. 
•00141 ~ AmTl” “ “ b cm. sec. 
The temperature of the air in this experiment was 17° C. hence we have for the 
thermal conductivity of the brass bar used* 
k = ‘268 (1 + ‘002 v - 17°) 
cst gram. 
cm. sec. 
Experiments on Crystals, &c. 
Before cutting the bar for the insertion of the discs the conductivities of which 
were to be measured, several observations were made of the distribution of tempera¬ 
ture along the bar after it had been painted with Aspinall’s enamel. It is necessary 
to have the radiating surfaces of bar and discs the same, and this is most easily 
secured by painting both. The loss of heat from a point of the surface at any 
temperature is about 40 per cent, greater than the loss at the same temperature 
for the nickel-plated surface, but it does not increase as rapidly with rise of tempera¬ 
ture. On this account it is possible to express the distribution of temperature along 
the bar in the empirical form v = Asinb (ax + B), and A, a, and j3 can be determined 
so that this equation holds with a closer approximation than in the case of the 
unpainted bar, throughout the whole length of the bar. It is assumed to hold in 
what follows for each half of the divided bar, a having the same, but A, /3 having- 
different values for each half. 
The bar was divided in the middle, between the contact holes 1 cm. apart, and the 
ends ground down to be as nearly as possible planes perpendicular to the axis of the 
bar. To secure this a vertical hole, a little larger than the bar, was drilled in a prism 
* The following values of Tc for brass have been found by different experimenters : —Neumann, ‘ Ann. 
de Chim. et de Phys.’ (III.), vol. 66 (1862), - 302 ; Weber, ‘ Monatsber. Berlin Akad.,’ for 1880, p. 457, -150 ; 
Lorenz, ‘Wied. Ann.,’ vol. 13 (1881) ; ‘red brass,’ '252 (1 + '0018 t .) ; ‘yellow brass,’ - 212 (1 + -0020 t.). 
