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IOWA ACADEMY OF SCIENCE 
cal experiments using pressure to secure contacts, it is impos- 
sible to get results that can be duplicated, and it was deemed 
inadvisable to use Mercury contacts because of the uncertain 
action of it on Tellurium. This difficulty of contacts was elim- 
inated by a method of welding Tellurium crystals and Platinum 
wires. The process is rather delicate, but crystals as small as 
.02 cm. in diameter have been used, with Platinum wire .015 cm. 
in diameter. These fine wires bend very readily and relieve any 
small strains before the crystal is stressed to the breaking point, 
and because of this fact it was found to be inadvisable to use 
larger Platinum wires because of their stiffness. The crystals 
were put in circuit by welding the free ends of the Platinum to 
Copper wires. 
Fig. 13. — Curve showing resistance of tellurium crystals. 
For temperature adjustments in producing the curve shown 
in figure 13, the crystals were immersed in paraffin, and the 
experiments taken from room temperature to 380 degrees centi- 
grade. From this curve two temperature coefficients are ob- 
tained, for use in the formula R*— K® (1+at). The first is from 
20° C. to 150° C., and is aj,— — .00584. The second is from 200° 
C. to 400° C., and is ao= —.00298. 
In obtaining the curve, figure 14, the same, sort of mounting 
was used as before, except that the crystal remained in air. 
The peculiar shape of the E-I curve is due entirely to the nega- 
tive heat coefficient in the resistance formula. No attempt was 
made to regulate the temperature, and the heating was due to 
