File:Methods for close automatic control of incubating temperatures in laboratories (1921) (14781576515).jpg

Identifier: methodsforclosea951bowe (find matches)
Title: Methods for close automatic control of incubating temperatures in laboratories
Year: 1921 (1920s)
Authors: Bowen, John T. (John Thomas), 1873-1940
Subjects: Laboratories Tuberculin Temperature control
Publisher: Washington, D.C. : U.S. Dept. of Agriculture
Contributing Library: U.S. Department of Agriculture, National Agricultural Library
Digitizing Sponsor: U.S. Department of Agriculture, National Agricultural Library

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tlyacross 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 thetemperature in the room slightly below the point desired. The othertwo coils, which are connected in series, furnish the additional heatrequired to bring the temperature of the room up to the desired point.The latter two coils are connected to and operated by the controllingsystem. These coils are connected through the relay to the other sideof the 220-volt line through a variable resistance in series with theinstrument. This resistance is sufficient to reduce the voltage in 8 BULLETIN 951, U. S. DEPARTMENT OE AGRICULTURE. the relay circuit to four volts, and thus prevent arcking and burningthe thermostat and relay contacts. Condensers are placed in thecircuits where shown (fig. 6) for the purpose of absorbing the induc-tive discharge from the coils when the circuits are broken.
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Fig. 5.—Temperature-control board of incubating room in Patbological Division. The relay is connected for reverse action; that is, when the electriccircuit is completed through the thermostat, owing to rising tempera-ture in the room, the relay operates to break the main heating-coilcircuit and thus cut off the supply of heat. On the other hand, whenthe relay circuit is broken in the thermostat, owing to a lowering 0 CONTROL OF INCUBATING TEMPERATURES IN LABORATORIES. 9 temperature in the room, the rela)^ closes the heating-coil circuit.The overload cleAdce, shown in the diagram, has for its object the cut-ting out of the whole system in case the temperature-control appa-ratus fails to oj^erate. In principle it is identical with the tempera-

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