REGULATION OP HIGH TEMPERATURE OVENS 
301 
A DESIGN FOR ELECTRICAL REGULATION OP HIGH 
TEMPERATURE -OVENS. 
W. E. TISDALE. 
In' growing Tellurium crystals, a temperature within a few 
degrees of 450 centigrade must be maintained constantly. The 
electrical oven available consisted of a porcelain tube five centi- 
meters in diameter and thirty centimeters long, covered with 
asbestos. The need for making a regulator arose from the fact 
that nowhere in the catalogues available was there a regulator 
advertised that would be contained within the tube without 
completely closing it. Nearly all temperature regulators for 
electrically heated apparatus break the heating current when 
the temperature rises to a given point, and make it when' the 
apparatus cools, and the difference between these two tempera- 
tures is the regulation of the device. It requires 10 amperes 
to heat this particular oven to 500 degrees centigrade, and sim- 
ple expansion could not be relied upon to make a gap suffici- 
ently wide to prevent sparking and at the same time give any 
very close regulation. 
Any device using electricity to break the heating circuit must 
operate with a very small current because of sparking across 
the' gap made by the contraction of the contacts. This con- 
traction should be as great as possible, and in the oven for which 
this device was made there was not sufficient room to use levers 
to increase the gap. A mercury-in-glass device was first used, 
in which the circuit through a storage cell and relay was com- 
pleted by the rising surface of the mercury coming in contact 
with a platinum point. This was discarded because the oxide 
and vapor formed at each contact made it necessary to keep 
vigilant watch to assure its working. Another device consisting 
of a strip of brass and copper riveted together, and using the 
unequal expansion of the two to bend the strips and make con- 
tact against a suitably placed point was discarded because of its 
lack of fine regulation. The present device utilizes the geometric 
principle that if the base of a right triangle be kept constant, 
the perpendicular increases more rapidly than the hypotenuse,, 
and especially for small increments. 
