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INDUSTRIAL AND ENGINEERING CHEMISTRY
obtained. Frequently, "rated" or "replacement" values are given for paper condensers which are much greater than the actual effective capacities. Potentiometer RI should also be of good construction, so that it will repeat its setting and will not be subject to sudden and unpredictable resistance variations. The relay, Re, should be carefully selected, since many of the cheaper varieties are not reliable and may fail to make contact at times because of poor mechanical construction.
The operation of the circuit is simple and may be described by folloning through a complete cycle. Let us start at that point in the cycle where a plate current has just started t o flow and before the relay has had time to close. The thyratron acts as a half-wave rectifier. Because of the current in the plate circuit the relay coil is energized and there is also a potential drop across the relay coil and R4 as indicated by the signs of Cz. As soon as the relay contacts have closed, this potential drop acts as an electromotive force in the circuit containing condenser Ct, the terminals of which are connected to the grid and cathode of the thyratron. As a consequence the condenser is charged and the potential of the grid becomes decidedly negative with respect to the cathode. This negative potential of the grid has no effect on the plate current during the time the thyratron is ionized, However, it prevents reionization following the deionization which occurs during that half of each current cycle in which the plate is negative. The rectified current, therefore, ceases and relay contacts PI and PZopen, leaving condenser Ct charged. This charge then leaks off through R1 and Ra according to Equation l, and as it does the potential of the grid becomes less negative. After the critical value is reached the tube reionizes as soon as the potential of the plate reaches a peak in the positive direction, thus completing the cycle. The relay is de-energized most of the time. It receives short pulses at time intervals determined by grid circuit elements C1, R1,and RP. If it is desired to have current flowing in the load circuit during the discharge interval instead of a momentary pulse,
Vol. 14, No. 5
the "back contacts" of the relay may be used. The values given for R1,Rn,R4,and C1 provide a time-interval range from 140 to 9 pulses per minute over the range of adjustment of potentiometer Rt. Resistance R2determines the minimum time interval available. Changing Ra will change the time-interval range because of changes produced in the charging voltage, E. If a greater range is desired, a group of resistors may be used with a selector switch as shown by Goldberg (S), or other condensers may be switched into the circuit by similar means. The relay indicated will carry currents up to 5 am eres at 110 volts through its contacts. If the controlled l o a f requires a p t e r current, other relays may be selected or a second relay of igh current-carrying capacity may be operated from points PI. Either alternating or direct current may be used for load operations. I n the particular application shown in Figure 1 the controlled load was actuated by current from a 110-volt direct current source. Condenser Csis used to prevent arcing a t oints PI. This condenser should be selected to have the best vake for the particular reactance and resistance characteristics of the controlled load.
The apparatus described here may be applied to a wide variety of timing operations. For time intervals not available with the components shown in Figure 1 it is only necessary to replace R1, RP,and C1with components having the desired values which may be calculated from Equation 1.
Literature Cited (1) Electronics, Reference Charts, 14,No. 6 (1941). (2) Gilson, W.E., Photo Technique, 2, Part 2, 28 (1940). (3) Goldberg, H., Ibid., 3,Part 1, 56 (1941). (4) Mucher, G., Electronics, 9, No. 4, 38 (1936). ( 5 ) Smiley, G., Ibid., 14,X o . 1, 29 (1941). PAPERNo. 41 of the Portland Cement Association Fellowship a t the National Bureau of Standards.
Design for Rectifying Column E. 0. RAMLER AND J. H. SIMONS School of Chemistry-and Physics, The Pennsylvania State College, State College, Penna.
FP10"
rectification of boiling in the range -60" C., there is frequent need for a smalllow-temperature column which can be readily moved about the laboratory or put away when not in use. Such a column has been described for materials boiling around -50" C. or below, but vhile simple to operate, it is not easy for the average amateur glass blower t o construct. The low-temperature column here described is a simple modification of that described by Simons ( f ) , and has been used successfully in this laboratory with liquids boiling at -30" and -5" C. I n Figure 1 is a diagram of the column, which is about 45 cm. (18 inches) in height and fits conveniently into an ordinary quart vacuum flask. A is the liquid container with a volume of about 50 cc. Sealed on the bottom is the nipple, B, which is wound with a heating coil of asbestos-covered Nichrome wire, B. & S. S o . 26. The packed section,,C, of the column is 9-mm. tubing and contains glass helices supported by a small glass cross bar. D is the take-off tube in which is sealed a small thermocouple well, contaming a copper-constantan couple. The condenser, E , consists of two concentric tubes of 42and 37-mm. outside diameter, sealed at the top. The 7-mm. tube, F , sealed at the top
of the condenser, is to permit the escape of air as the material is condensed in A , The fractions may be obtained by the use of a three-wav StODcock on D. or of a chain of traDs in series. The column is very simple to operate. It is placed in a quart-size unsilvered Dewar flask, which is surrounded by a radiation shield of aluminum sheet containing slots cut into it for observation of the pot and reflux. Leaving tube F open, the material is condensed in A through the take-off tube with the aid of the cooling mixture in E and in the Dewar flask. The Dewar flask is now removed, emptied, and replaced around the column. With D closed, A is heated if necessary to bring about a reflux, When equilibrium is established, F is closed and D is opened, and the condensate is received in a series of traps, the last one of which is open to the air. If the take-off is too slow, it may be increased by the use of a very slight vacuum obtained from an ordinary laboratory watersuction pump, applied to the last trap through a stopcock. The amount of take-off can be regulated by adjustments of both the heat supplied to the pot and the vacuum on the receiver. The temperature a t the top of the column can be determined with a potentiometer.
Literature Cited FIGURE1
(1) Simons, IND.ENQ. CHEM.,ANAL.ED.,10, 648 (1938).
