42 
GEORGE F. POTTER 
Regulation of the apparatus is accomplished by means of an adjusting 
screw at the top of the connecting rod {A, Plate II). The temperature 
within the chamber is read with a thermometer. The drum is revolved until 
the long arm indicates the same temperature on the chart. The screw is 
then turned until the platinum point just makes a contact with the mercury 
in the capillary. In making this adjustment it is necessary to allow for 
some ''lag" in the thermostat if temperatures are changing rapidly in the 
chamber and if the thermometer used is a sensitive one. As a rule the 
apparatus is held about fifteen minutes at the adjusting temperature. In 
operation, the ''lag," using the large thermostat mentioned above, did not 
cause variations in temperature of more than 0.25° C. when the temperature 
of the chamber was changing at a rate of 16° C. per hour. 
The connecting rod is so designed that it may be lifted up and detached 
from its support on the short arm of the revolving shaft. The base of the 
clock mechanism is fastened to a slotted support by a screw clamp. By 
detaching the connecting rod and loosening the clamp, shaft and clock can 
be removed. The insulated wire leading to the thermostat is then dis- 
connected from its binding* post and the covers of the freezing chamber may 
be removed. 
The use of this machine has enabled the writer to perform freezing ex- 
periments under conditions controlled more accurately than has been 
possible even with the closest personal attention when using a hand-con- 
trolled freezer. The range and accuracy of regulation of temperature 
depend almost entirely upon the thermostat. The instrument used by the 
writer works through a range of 10° C, and the variations between th^ 
temperature indicated on the chart and that observed in the chamber usually 
are not greater than 0.1° C, although sometimes variations of 0.25° C. are 
observed. A less sensitive thermostat capable of working through a corre- 
spondingly greater range of 40° C. showed maximum variations of about 
0.5° C. However, if particles of mercury become separated from the top 
of the column in the capillary tube, the operation of the machine becomes 
unreliable and usually the temperature of the chamber becomes too high. 
To avoid this the capillary must be cleaned of any oil or dirt about once a 
week, or sometimes oftener if foreign materials chance to enter. It is 
also necessary to keep the glass guiding rod of the contact point {CC, Plate II) 
above the surface of the mercury because if immersed it breaks the column, 
carrying part of the mercury up above the rod. It is possible that this 
difhculty could be eliminated by the use of some other type of thermostat, 
but the writer has not been able to find one which has the same sensitivity, 
and at the same time the ability to return accurately to the original starting 
point after going through wide temperature variations. The last men- 
tioned characteristic is absolutely essential for the operation of a machine 
of this sort. Even with these limitations the machine enables the experi- 
menter to do much more accurately controlled work than would be possible 
