A method of automatic control of LOW TEM¬ 
PERATURES EMPLOYED BY THE UNITED STATES 
DEPARTMENT OF AGRICULTURE 1 
By John T. Bowen 
Technologist , Bureau of Animal Industry , United States Department of Agriculture 
In many scientific investigations the automatic control of low tempera¬ 
tures between narrow limits is very essential. Accurate automatic 
control of high temperatures is comparatively simple with electricity 
employed as the heating medium, but close control of temperature below 
that of the surrounding air is attended with more or less difficulty, for 
the reason that some form of refrigerating machine is necessary for remov¬ 
ing the heat and it is difficult to compensate for the inherent lag present 
in machines of this character, and also in the thermostatic elements 
which are necessary for turning on and shutting off the supply of the 
cooling medium. In addition to the inherent lag of the refrigerating 
machine and thermal regulator, in certain work, as in studying the effect 
of low temperatures on plant life, where it is necessary that the plants be 
exposed to the sunlight, the heat from the sun adds greatly to the diffi¬ 
culty of accurately controlling the temperature. Furthermore, this 
difficulty is greatly augmented by passing clouds which suddenly shut 
off or let on this source of heat. The heat from the sun is about 7 B. t. u. 
per square foot per minute at the upper limit of the atmosphere. A part 
of this heat, however, is absorbed by water vapor, dust, etc., contained 
in the atmosphere; consequently, about 5 B. t. u. per square foot per 
minute is delivered on the earth’s surface in the vicinity of Washington, 
D. G. Passing clouds reduce this supply of heat to perhaps 2^ B. t. u. 
per square foot per minute. It is obvious, therefore, that the problem 
of maintaining an approximately constant temperature is a difficult one 
and requires great care in order to secure satisfactory results. In order 
to minimize the thermostatic lag the instrument should have as small 
heat capacity as possible, so that it may respond quickly to slight changes 
in temperature. To procure a thermostat suitable for this particular 
work it was necessary to design one. In designing the instrument the 
object was to produce one that would show the same lag effect as the 
standard measuring instrument, the Beckmann thermometer. The instru¬ 
ment was designed by Dr. R. B. Harvey, and a description of it appeared 
in the Journal of Biological Chemistry, in January, 1920. 2 
Where the problem is one of accurately controlling the temperature 
only, it is an easy matter to cool the air by refrigerating well below the 
desired point and then heat back by electricity. By controlling the elec¬ 
tric current the temperature may easily be maintained practically con¬ 
stant. In most cases, however, the relative humidity of the air is as 
important as the temperature, and should the temperature of the air be 
reduced below the dew point a part of its moisture will be deposited on 
1 Accepted for publication Aug. 3, 1923. 
a Harvey, R. B. a thermoregulator with the characteristics or the beckmann thermometer. 
In Jour. Biol. Chem., v. 41, p. 9-10, pi. 1. 1920. 
• Journal of Agricultural Research, 
Washington, D. C. 
(183) 
Vol. XXVI, No. 4 
Oct. 27, 1923 
Key No. A-6s 
