CONSTANT-TEMPERATURE BATH EMPLOYING THERMIONIC CONTROL G. W. THIESSEN
AND
L. J. FROST
Monmouth College, Monmouth Illinois
C
ERTAIN conductivity work in progress in this laboratory necessitated the use of a constanttemperature bath. As no ready-made bath was available, one was made from odds and ends which may be found in almost every laboratory. .Although some of the steps in the assembly may appear rather complicated at first glance, the work is not beyond the capacity of amateurs and good results are obtained. The bath itself was a six-gallon glass aquarium. The electrical equipment was constructed from parts either at hand or carried in stock by the average radio shop. The relay, type R6, made hy L. S. Brach, Newark, N. J., was modified by substituting 12,000 turns of No. 36 enameled wire with a resistance of 11,000 ohms, for its original winding, and connecting the points so that the breaks occurred 'in series. The relay, together with two radio tubes, typv CX301-A, two 50watt electric light bulbs, and a 25 m. f. d. condenser made by Polymet Mfg. Company, New York City, was mounted on a board eight by sixteen inches. Suitable terminal connections were made for the power line, thermoregulator, and beater. With excessively low line voltage, one or both bulbs may be 60 watts. The action of the vacuum-tube thermoregulator circuit is as follows: With contacts, T , open, current flows through V, 1, h, Vl, in series, lighting the filaments. When A is positive, the tube Vl is functioning with about 50 volts positive potential applied from B to its plate through the relay R. This is sufficient to energize R and attract its armature, holding the heater circuit closed during the half-cycle. Tube V is meanwhile inoperative, merely acting as a resistance in the filament circuit of the operative tube. When B becomes positive in the next half-cycle, V and VI interchance functions, with no reversal or interruption of curreit thrthrough R. Now when the thermore&ator T
closes its circuit (through S,a 10,000-ohm resistor), the grid of each tube as it operates experiences a large positive bias, reducing the flow of electrons from filament to plate to such an extent that the current through R is no longer sufficient to hold the armature down. The heater circuit is thus opened. The condenser F serves to steady the action of R. The heater' was made from three-eighths-inch pyrex glass tubing bent into the shape of a U. This tube was
filled to a height of about 35 cm. with fine, tightly packed graphite. Connections were made to the graphite with large copper wire which was pressed tightly against it. It is essential that the graphite be tightly packed and that the connections be good and fim. Also it may be noted that an electric ligbt bulb submerged in the bath is a very satisfactory heater when the light is not troublesome and the desired temperature is low. A lamp beater can be improvised by soldering two copper wires onto an electric light bulb as suitable leads. A large rubber tube is placed over these wires onto the base of the bulb. Long glass I. A. COWPBRTHWAITB, I.Am. Chem. Soc., 49,2255 (1927).
tubes encase the two lead wires and are inserted into a lary. The regulator was then ready to be filled with two-hole stopper. This stopper in turn is forced into mercury. This was done by means of a funnel fitted the l a c e rubber tube which has been made fast to the with a capillary fine tip. The regulator was filled to within eight inches of the base of the bulb. The joints may be too. Then the mercurv connection was made with wired with copper wire to effect waterthe sealed-in wire and a lead was run from it to the tight connections. A 6fty- to hundredterminal indicated in the diagram. Into the capillary watt bulb has been found practical. An auxiliary heater is desirable for end of the thermoregulator a stiff wire was forced (chromel or s t 3 iron wire is a good choice). When the initial heating and may be made contact with the mercury is made, the heater in the as follows. A five-inch porcelain insucircuit is shut off. Thus the desired temperature is lator or equal length of one-half-inch obtained by pushing the wire in or pulling it out. Some pyrex glass tubimg is wrapped with 50 of the mercury may have to be run out for high temturns of No. 28 chromel wire, care being peratures and this may be done by heating the bulb and taken that the wire of one turn does not knocking off drops of mercury until the right height is touch that of the next. This heater may found. It is helpful to hang the regulator loosely, as on be suspended in the bath on leads of springs. heavy copper wire and connected diThe advantages of this device over simpler ones not rectly tothe 110-volt circuit when needed. employing the tube control lie in the operation of the The thermoregulator was made from mercury meniscus on almost an open circuit, the avoida one-by-eight-inch test-tube sealed to ance of all use of batteries in the operation of the a capillary tuhe twenty to twenty-four reeulator, the eain in steadiness resultina from relavinches lona and havina a bore about the size of a ko. 28 wiri. The smaller the capillary the inregration of tge cyclic action of the current, and the more sensitive the regulator, but it must be remembered efficient use of power consumed by makimg the filament that No. 28 wire must go in the end of it. Next a wire lighting circuit part of the potential divider used to was sealed into the end of the test-tube near the capil- apply the proper potentials to the plates. lary. This made a permanent contact with the inside of the bulb of the regulator. This wire was then enAcknowledgment is made of the help of Mr. Mearl closed in a one-eighth-inch tuhe parallel to the capil- Tilden, a:radio engineer, in this work.
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