The change is very rapid for calibration steps of 1 
to 5 degrees because of the large reserves of hot 
and cold water available instantly on demand. The 
hot tank contains 7 gallons maintained at 80°C, 
and the cold tank 25 gallons maintained at —3°C. 
Step 3: Insert Thermometers 
Twenty-four thermometers are mounted in the cir- 
cular rack and the whole lid is lowered, so that the 
thermometers are wholly immersed and the aperture 
sealed. The stirrers are then placed in operation to 
give very rapid mixing of the water mass and as 
nearly homogeneous a temperature structure as is 
possible. 
When the desired temperature has been attained, 
the metering valves are adjusted to make the flow 
of water from the hot or cold tanks exactly compen- 
sate for unavoidable transfer of heat from the tank 
because of ambient differences of temperature. An 
extremely small aperture is needed to avoid over- 
shooting because of the slight lag in the thorough 
mixing of the incoming water with the whole water 
mass. Metering is made possible by the use of cen- 
trifugal pumps which act as their own bypasses when 
the flow is shut down. This system was found to be 
simpler and more readily controlled than that of 
varying the speed of the motors driving the pumps, 
which was tried at one stage of construction. 
Step 4: Reverse Thermometers 
Ordinarily the thermometers are left for 10 minutes 
at a given temperature to assure full equalization. 
During this period the bridge operator constantly 
watches the galyanometer spot. If the spot varies 
more than 1 mm (i. e., 0.001°C) from its null, the 
operator makes the necessary adjustments of the 
metering valyes. Several minutes of stabilization are 
required after each temperature change. 
After the temperature has remained constant for 
the required period, the operator at the controls 
reverses the rack so that it locks in the reading posi- 
tion. If the mercury columns in any of the thermom- 
eters are found not to have broken, the lid may be 
raised a few inches and the rack tapped sharply 
without changing the temperature appreciably. 
Step 5: Read Thermometers 
The operator at the telescope can now read the 
main thermometer of each unit successively by oper- 
12 
ating the indexing mechanism which rotates the rack 
to move successive thermometers into reading posi- 
tion, either clockwise or counterclockwise. There is 
a 7-second pause between moves, which is usually 
sufficient for reading. 
There ordinarily is no need to read the auxiliary 
thermometer because the surrounding bath is main- 
tained at constant temperature by the operator at 
the temperature controls. Of course, if required, 
calibration can be established for auxiliary ther- 
mometers at the same time. This is done by having 
both operators read the thermometers. Then the 
whole process is repeated twice at the same tem- 
perature, resulting in three double checks at each 
temperature check. This is not only a check on accu- 
racy, but also gives a very good evaluation of the 
dependability of the individual thermometers. 
Since the thermometers are read at the same tem- 
perature at which they were reversed there is no 
need to go through the tedious temperature correc- 
tion required when they have to be removed from 
the bath and allowed to reach a steady room tem- 
perature, which in itself usually takes at least 30 
minutes. 
No time is lost waiting for thermometers to equal- 
ize after they have been reversed. And, of course, 
there is no correction for thermal expansion of the 
mercury. The index correction is read directly, thus 
saving time and possible errors. 
It is estimated that with two experienced oper- 
ators it is possible to calibrate a set of twenty-four 
thermometers in two days, depending upon the range 
of the thermometers and the number of points (usu- 
ally every 2°C) to be read. Thus, the tank enables 
thermometers to be calibrated accurately and surely 
against a known standard in about one-fifth the 
time previously required and with about one-fifth 
of the labor. 
CONCLUSIONS 
Preliminary use and evaluation of the temperature- 
controlled tank indicate that if gives an accuracy 
of +0.002°C over a range of temperatures from 
—3°C to 40°C and provides great savings of time 
and effort in calibration of reversing thermometers. 
The accuracy is dependent upon the reliability of 
the Leeds and Northrup platinum resistance thermom- 
eter, the Muller bridge, and the thermostat. The 
system is checked periodically for ice point and the 
