CONTROL OF INCUBATING TEMPERATURES IN LABORATORIES. 7 



designed by the bureau demon:^trated on tests that the temperature 

 inside the room coukl 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 electricall}^ 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 comers. 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 26 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 aetualh' 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 beloAv the point desired. The other 

 two coils, Avhich 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 



