Three recent articles (1, 2, 3) describe methods of heat control which seem unnecessarily complicated in view of the present technical development of control tubes in the radio industry. Those methods, just mentioned, use mechanical relays and have wiring schemes which, although not complicated to those acquainted with radio, are nevertheless too difficult for many people to construct. One of the simplest and most effective schemes involving the use of thyratron tubes and no mechanical relays was described by SCHWENK and NOBLE (4) in 1937. The scheme here described is a further simplification and adaptation of thyratron methods.Recently two well-controlled incubators were needed and were obtained by converting two old ovens that had long been around the laboratory unused. These ovens had been controlled by the usual mechanical thermostat and allowed a temperature variation of + 20 C. The conversion of the ovens to valuable incubators was done as follows:' A five-volt transformer was mounted on top of the oven with the primary connected to the 110-volt line and the secondary connected to the two large prongs of a four-prong socket as filament supply for the control tube. Then the two heavy wires (the 110-volt line for the heater) that had led to the mechanical thermostat were disconnected and one of them soldered to the two small prongs (cathode and shield grid) of the four-prong socket. The other wire was soldered to a clip to attach to the tip (anode) of the control ttbe. The tube is now wired with plate and filament voltage and all that remains is to connect to the new thermostat. Any type of thermostat can be used. For the ovens just mentioned, coiled bimetallic thermostats were taken out of ten-cent store thermonmeters. The thermostat was left mounted on the metallic dial of the thermometer, but the hand was broken off and a short shaft soldered in its plaee. An insulated contact was mounted so the free end of the thermostat could touch it. The thermostat was then mounted inside the oven with the shaft sticking through the top of the oven. An ordinary dial was attached to the shaft. Adjustment for any range is obtained by turning this dial which winds up or unwinds the bimetallic thermostat. The wiring is completed by attaching one contact of the thermostat directly to the control grid, while the other one is connected to the anode through a 10-megohm resistance.Full-sized blue prints giviaig all details of construction are available for 25 cenits.
