A CONVENIENT THERMOREGULATOR 
HEINRICH HASSELBRING 
(WITH TWO FIGURES) 
A convenient form of thermoregulator which has now been in 
use in several laboratories of the Bureau of Plant Industry, United 
States Department of Agriculture, for nearly ten years is shown 
in fig. 1. On account of its compactness and its adaptability to 
constant temperature baths and chambers of various kinds, where 
extreme sensitiveness is required, it may perhaps be worth while to 
give a brief description of the instrument and its installation in 
order to make it more generally available. 
The instrument consists of a thermometer tube about 30 cm. 
long, the upper end of which is bent over and enlarged into a bulb 
to serve as a reservoir for excess mercury. ‘The thermometer bulb 
at the bottom should be about 7 or 8 cm. long. Platinum contacts 
leading to the binding posts clamped on the thermometer are 
sealed into the capillary at A and B. Ata point (C) some distance 
from the upper contact a slight constriction is formed in the 
thermometer capillary. This point may be marked on the tube. 
For ordinary incubator temperatures the constriction should be 
about 2.5 cm. above the upper contact; for temperatures around 
zero it should be 5 or 6 cm. above. 
Experience with many of these instruments has shown that in 
their manufacture the observance of several points is absolutely 
essential. (1) The space above the mercury must be entirely free 
from gas. The presence of a trace of gas prevents the union of 
mercury from the reservoir with that of the capillary and makes 
the instrument entirely useless. (2) The platinum wires must 
project far enough into the capillary to make contact with the 
mercury, but not so far that they interfere with the motion of the 
mercury column. If the wire projects too far into the capillary 
327] [Botanical Gazette, vol. 71 
