THE THERMAL CONDUCTIVITY OF TELLURUM 
215 
b. The heat was supplied by a small heating coil while the cooler 
was connected to the water supply. The E. M. F. ’s were meas- 
ured with a potentiometer and since these were directly propor- 
tional to the temperatures the ratio K/K, was obtained by using 
them in place of e 3 , e 2 , and e x . 
It was at first intended to compare the tellurium disk directly 
with a lead disk but in some preliminary work with lead and 
tin using lead as a standard the results for tin were consistent 
but were about one-third of the value given by the latest tables. 
By increasing the ratio of the surface to the area of the edges, 
the values became more nearly those of the tables. This indi- 
cated a constant error due to end losses which the guard ring 
did not prevent. Although the guard ring did not prevent end 
losses it insured constant equilibrium conditions and as these 
were more important in the plan used than prevention of end 
losses the guard ring was maintained. The plan used was as 
follows. Let K g , Ki , K sn , Kt denote the respective conductivi- 
ties of glass, lead, tin, and tellurium. First we may compare a 
lead disk with a glass disk. We find: Ki /K g =A. Also com- 
paring tellurium with glass :Kt /K g = B. 
By dividing these equations as they stand we obtain : 
K, /K t =A/B. 
In a similar way we may find the ratio K sn /K t = A t /B. 
or K sn /K] —A 1 / A. 
This latter test of lead and tin agreed with the tables to within 
11 per cent. Considering that this method was designed pri- 
marily for poor conductors these results can.be considered very 
satisfactory. Different thicknesses of lead, and glass disks were 
used with the tellurium and the agreement of the results was 
sufficient to point, to the conclusion that, the value of K t obtained 
was in the neighborhood of the true value. The combinations 
(thickness expressed in millimeters) and the ratios were as fol- 
lows : 
SET. 
I. 
Pb. 
2.47 
glass 
.93 
K x 
/ Kg 
= 13.1 
Te. 
2.24 
glass 
.93 
K t 
/Kg 
= 1.41 
K, / Kt = 
= 13.1 
/ 
1.41 
— 9.3 
SET 
II. 
Sn. 
2.51 
glass 
.93 
• 
K§n A Kg 
= 22.8 
Te. 
2.24 
glass 
.93 
; 
K t 
/ Kg 
= 1.41 
Ksn/ Kt = 
= 22.8 
/ 
1.41 
= 16.2 
