A N A L Y T I C A L CHEMISTRY
168 a boosted signal corresponding to the difference between the setting of the Rubicon Type B potentiometer and the e.m.f. of a Chromel-rllumel furnace thermocouple. The recorder is equipped with a cam, relay, and resistor8 so that it controls the temperature that it measures. By this arrangement-essentially zero suppression-it is possible to record temperature over a wide range without loss of sensitivity. It was found that room temperature changes overnight were sufficient to change the potentiometer working battery voltage by more than 1mv., or the opposition voltage by more than 12 pv. (equivalent to a temperature change in the furnace of about 0.3” C.). The servo system monin
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Figure 3.
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SHUNT RELAY
Schematic Diagram of VoltageControlling System
tors and adjusts the 6-volt supply by comparing it continuously with an Eppley standard cell (unsaturated type). The experience of the authors is that after 16 hours of unattended operation the working supply voltage seldom differs by more than 50 pv. from the desired value, and is usually within 10 pv. of this value in spite of the fact that room temperature changes of 20’ F. are commonplace. .411 the components of this system, including the components of the servo, are commercially available. ACKNOWLEDGMENT
The authors are indebted to William L. Briscoe of this laboratory for the design of the servo amplifier employed. This work was performed under the auspices of the AEC. LITERATURE CITED
Dunn, F. J., Mosley, J. R., and Potter, R. XI., ANAL.CHEM.,27, 63 (1955). Farkas, A., and Farkas, L., Yature, 132, 894 (1933). Farkas, -4., Farkas, L., and Rideal, E. K., Ibid., 137, 315 (1936). Grenorv. H. S.. and Archer, C. T.. Phil.Mae. (7). 15,301 (1933). Neℓ W. C., Purcell, R. H., Gregory, H. S., and Ellingham, H. J. T., Nature, 137, 69 (1936). Reilly, J., and Rae, W. N.. “Physico-Chemical Methods,” Vol. I, p. 449, New York. D. Van Nostrand Co., 1943.
Water-Flow Safety Switch for Gas or Electric Heaters R. P. Harpur, Institute of Parasitology, McGill University,
the remainder of the system. Safety devices described include a thermoregulator sealed into the system for protection of electrically heated water stills (S),a pressure-sensitive relay ( I ) , and more recently, a bellows-type pressure switch ( 2 ) for use in conjunction with a mercury diffusion pump or other electricallv heated equipment The device described here is of simple construction and has been in use for some time with a mercuri- diffusion pump.
A Type W lever arm microswitch, S (normally closed and requiring 35 grams for opening), is mounted on the wall above a large glass test tube (3.5 cm.), which serves as a float chamber, as shown in the figure. .4length of S o . 16 copper wire is soldered to the switch lever and after bending is run through two corks (No. 8 and KO.IO). A piece of solder wire is placed around the copper wire above the corks so that its weight is just sufficient to operate the switch to the down or open position. The return water from the condenser enters the float chamlicr by the inlet tube, A , and leaves by the drain tube, B, a t a rate determined by the setting of the pinchcock. The pinchcock is adjusted to pass water at a rate below that desired for cooling the system. When the rate of water flow increases beyond this amount, the water level rises to the overflow tube, C, and therehr raises the float to switch on the heater. Conversely, a decrease in water flow xi11 switch off the heater, and if a double throw type of switch were used an alarm could be operated. There i- an automatic check every time the still is switched on, as power is not supplied to the heater until the water has been turned on. Furthermore, should the cork float become waterlogged after long continuous use, power failure rather than failure to witch off would result If a separate pilot flame is used, gas burners (using butane in this laboratory) may he controlled electrically by the use of x relatively inexpensive and commercially available magnetic gas valve (Minneapolis-Honeyv, ell 1‘495A). Thus, an electric safetv switch may be used for gas or electrical heating. 4CKKOW LEDGMENT
The financial assistanre of the Kational Research Council of Canada is gratefull\, acknowledged
Macdonald College, P. Q., Canada
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LITERATURE CITED
the dependence of many laboratory operations upon water cooling of condensers, there exists the hazard of water pressure failure. If the condenser is part of a mercury diffusion pump, insufficient cooling may permit the mercury to distill into ITH
(1) Poole, J. H. J., and Gilmour, J. C., J . Sei. Instr., 20, 19 (1943). (2) Roth, W., A N ~ LCHEZI., . 25, 1940 (1953). (3) White, L. hl., and Secor, G. E., IND.ENG.CHEV.,A X I L .ED.,18,
332 (1946).
