window, which measures 9 by 19 inches, is a ther- 
mopane unit made to order by the Libby Owens 
Ford Glass Company and consists of four panes of 
Y4-inch, optically plane glass, opaque to infrared, 
mounted parallel in a metal-sealed frame. The three 
dead-air spaces between the panes give undistorted 
visibility with extremely low heat transfer. Twelve 
rubber-faced pressure screws hold the unit in place 
against an O-ring seated around a suitable aperture 
cut in the inner lucite shell of the tank. These screws 
are spaced evenly along its sides and upper and 
lower edges, and are mounted in aluminum brackets 
screwed to the frame of the tank. The frame for this 
window, which extends through the tank insulation, is 
made of wood with a removable rubber-lined inner 
frame, to give a snug positive fit for the thermopane. 
Reading Telescope 
To read the thermometers accurately, a reading 
telescope is provided. This is a 20X erecting instru- 
ment with horizontal hair at the principal focus of 
its eyepiece systems. It can focus to a minimum dis- 
tance of 10 inches, and at the 13-inch distance to 
the mounted thermometers its field is about 1 inch, 
which makes it possible to read the main and aux- 
iliary thermometers without moving it laterally. To 
assure accuracy, this telescope is held by a heavy 
frame, which moves laterally on ball bearings on 
two horizontal steel tubes pinned rigidly at either 
end to aluminum travelers. The telescope thus has 
a 4¥2-inch lateral traverse. The supporting travelers 
are in turn mounted on vertical steel rods on which 
they can be moved 18 inches vertically by means of 
rack and pinion gears. The mount may be locked 
in position in either of these movements by set screws 
actuated by means of knurled knobs. The supporting 
vertical rods are seated top and bottom in steel 
frames bolted to the front of the tank. This whole 
assembly is very carefully aligned so that the tele- 
scope is exactly perpendicular to the axis of the 
thermometers in all positions, thus eliminating error 
due to parallax, which is ordinarily the greatest 
single cause of error in reading thermometers from 
a distance. The thermometers themselves are held 
rigidly vertical when their rack is locked in reading 
position. The 7-pound weight of the telescope as- 
sembly is counterbalanced by two constant-tension 
negator springs so that it will remain in any desired 
position. 
10 
TEMPERATURE CONTROL UNIT 
Thermostat 
Temperature control is attained by means of a 
thermostat using two thermistor beads, one on each 
side of a resistance bridge (fig. 7). The other two 
sides of the bridge consist of a fixed resistor and a 
Helipot. The thermistor beads are mounted 3 inches 
to the left and at the same level as the thermometers 
to be calibrated. As temperature change occurs the 
resistance of the thermistors varies, throwing the 
bridge out of balance. This imbalance is fed to a 
dc amplifier. The amplified voltage drives a servo 
motor either clockwise or counterclockwise, depend- 
ing on the sign of imbalance. The servo, in turn, 
drives a cam mounted on a slip clutch that closes 
either of two microswitches. However, when exact 
balance is attained neither of the two switches (one 
controlling the hot and the other the cold transfer 
pumping system) is actuated. 
At full gain, this temperature-control system re- 
sponds more rapidly than can be measured with 
either the platinum or mercury thermometers. 
Control Panel 
The control panel is mounted on the left front 
supporting member of the tank. It can be swung into 
position for operation either by the thermometer 
reader at the telescope or by the operator at the 
temperature controls on the left (fig. 6). The panel 
contains the main power switch and controls for 
tank illumination, thermostat, pump, lid raise and 
lower, front and back stirrers, indexing and revers- 
ing. The temperature controls are necessarily on the 
panel of the thermostat, accessible to the operator 
of the temperature-measuring bridge. Illumination 
of the tank through the lucite inner wall is supplied 
by two fluorescent tubes mounted in recesses, one 
on each side of the thermopane. 
The final standard of temperature is obtained by 
means of a Leeds and Northrup platinum resistance 
thermometer and associated Muller Bridge, Type 
G-2, calibrated by the U.S. Bureau of Standards. 
This bridge measures resistance to 0.0001 ohm, the 
maximum correction given by the U.S. Bureau of 
Standards, thus enabling temperatures to be read 
to 0.001°C absolute. The manufacturer of the bridge 
claims an over-all accuracy only within several thou- 
sandths of a degree centigrade. However, in the 
