6 
BULLETIN 951, U. S. DEPAETMENT OF AGEICULTUEE. 
apparatus for the first incubating compartment was designed and 
for the most part constructed in the bureau. The temperature-con- 
trol S3^stem for the 
other compartment 
consists largely of 
assembled com- 
mercial apparatus. 
The heating units 
consist of resistance 
tubes or coils fed 
from a 220 -volt, 
direct-current line. 
The electric current 
is turned on andoii 
the resistance units 
by means of a ther- 
mostat in the incu- 
bator compartment. 
The thermostat is 
adjusted to turn 
the electric current 
on and off with a 
variation of room 
t emp e r a t ure of 1 ° 
C, that is, 0.5° 
above and 0.5° 
below the optimum. 
It is possible to ad- 
just the thermostat 
to turn the electric 
current on and off 
between much closer 
limits, but the 
limits of 0.5° above 
and 0.5° below the 
optimum tempera- 
ture give satisfac- 
tory results, and 
the frequency of 
operation and hence 
the wear on the 
controlling appara- 
tus are decreased. 
T h e temperature- 
control equipment 
