CONTROL OF IITCUBATING TEMPEEATURES IN LABORATORIES. 
7 
designed by the bureau demonstrated on tests that the temperature 
inside the room could be maintained between limits of ±0.1° C. 
In order that all parts of the room may be kept at approximately 
the same temperature it is necessary to provide means for thoroughly 
and continuously stirring the air in the room. This is accomplished 
by an electrically driven fan, as shown in figure 4, with the piping 
so arranged that the air is blown down over the heating coils in one 
corner of the room and drawn from near the floor at the opposite 
corner and from the top of the room at the other two corners. With 
this arrangement the air is stirred thoroughly and the temperature 
is maintained approximately the same in all parts of the room. Since 
it is necessary to operate the fan continuously it is belt-driven at 
about half-normal speed, or at a speed that will insure a thorough 
mixing of the air and hence a uniform temperature. By using an 
oversize fan and operating it at reduced speed the chances of a break- 
down of this important part of the equipment are greatly lessened. 
The fans used in these installations operate at a speed of about 
1,400 R. P. M. and deliver about 190 cubic feet of air a minute, or 
about 2G complete changes of air in the room every hour. The motors 
are shunt-wound, 230-volt, direct current. Two sizes of motors are 
used. Where a single motor drives two circulating fans a one-half 
horsepower motor is used, and where a motor drives only one fan a 
one-fourth-horsepower motor is used. The motor power is much in 
excess of that actually required to drive the fans ; but since they oper- 
ate continually and receive little attention, it is advisable to have 
considerably more power than is actually required. 
DESCRIPTION OF TEMPERATURE-CONTROL SYSTEMS. 
A general view of the temperature-control board, which was de- 
signed and constructed in the bureau, is shown in figure 5, and a 
diagram of the system is shown in figure 6. The electrical connec- 
tions covering the motor for driving the fan are simple and require 
no explanation. 
In the temperature-control system proper there are four coils of 
110 ohms each. Two of the coils are connected in series and directly 
across the main line, with a switch for cutting the coils out if desired. 
In cold weather the coils are brought into service. They are so pro- 
portioned that when continuously in service they will maintain the 
temperature in the room slightly below the point desired. The other 
two coils, which are connected in series, furnish the additional heat 
required to bring the temperature of the room up to the desired point. 
The latter two coils are connected to and operated by the controlling 
system. These coils are connected through the relay to the other side 
of the 220-volt line through a variable resistance in series with the 
instrument. This resistance is sufficient to reduce the voltage in 
