AN ELECTRICAL CONSTANT TEMPERATURE 
APPARATUS 
CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY I51I 
W. J. G. LAnp 
(WITH FOUR FIGURES) 
The temperature of incubators heated with gas taken directly 
from the mains is very irregular when an attempt is made to con- 
trol the flow with mercury-actuated thermostats commonly used. 
- Most mercurial thermostats will compensate for slight variations 
from the mean gas pressure, but not for large ones. The rise in 
temperature in the water-jacketed incubators used in the Hull 
Botanical Laboratory for paraffin infiltration was so sudden and 
so high that delicate plant tissues were often much distorted. 
Electrically controlled heaters have been placed on the market 
tecently by makers of repute, but the price of the apparatus ($50 
and $100) is unreasonably out of proportion to the cost of material 
and labor. 
In order to test the effect of definite temperatures on plant tissues 
for a longer time than is usually employed in imbedding, and having 
a direct current of 110 volts constantly on in the laboratory, the 
problem of devising a simple and efficient electrically controlled 
heater was first attacked about four years ago. The conditions of 
the problem were that the apparatus must maintain a definite 
temperature constant within very narrow limits for weeks at a 
time, must be easily adjustable to temperatures ranging from 
40° C. to 80° C. with certainty, must be absolutely automatic in 
action, must be readily attachable to the usual type of ovens, 
must require practically no attention to keep in order, must be 
simple and inexpensive to construct, must use a minimum quantity 
of electricity, and must not be easily put out of adjustment by 
inexperienced or meddlesome persons. 
For over two years the apparatus here described has replaced 
the gas heaters in this laboratory, with satisfactory results. The 
tisk of fire, always great when gas is used, has been eliminated. 
391] [Botanical Gazette, vol. 52 
