JOURNAL OF CHEMICAL EDUCATION
To the Editor: On page 556 of the October, 1951, issue of the
JOURNAL OF CHEMICAL EDUCATION, J. C. Hackney com-
ments critically on one of the series of exercises presented for illustrative purposes in my July, 1951, paper, "Evaluation of Achievement in Chemistry." While it is probable that on the level of high-school or elementary college chemistry the better students would respond to these items in the way intended, superior students whose understanding of oxidation, reduction, and neutralizatiou includes the concepts listed by Hackney could be in difficulty. His criticism admirably supports statements made elsewhere in my paper: "In concluding this discussion of various types of objective exercises, the importance of careful evaluation and editing of exercises prior to use should be stressed. After
JANUARY, 1952
Heater
exercises have been written they should be carefully checked for accuracy of phraseology and correctness of the key."
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To the Editor: The simple electronic relay described by John J. Pitha (J. CHEM.EDUC., 28,429 (1951)) is a device which fills a long-felt need. However, Mr. Pitha's use of a battery, which is bulky and needs occasional replacement, can be eliminated and the device made entirely self-contained. Indeed, the 3C23 tube works better when "triggered" with alternating current. At 120 volts on the cathode, 3 volts or more are needed for the grid bias. The most compact way of obtaining this would be to add an extra winding to the filament transformer. A few turns only of fine wire would he needed, as no more than 50 milliamperes is carried, and this could be poked in over the secondary without disassembling the transformer. It is more convenient, but slightly more expensive, to use a small auxiliary transformer giving about 6 volts. A bell-ringing transformer is suitable. The writer is using a t the present time a relay with the latter arrangement, the circuit diagram for which is shown in the figure. R1and R2are two ordinary halfwatt radio resistors with the approximate values of 20,000 and 100,000 ohms, respectively. The specifications are otherwise identical with Mr. Pitha's. The two transformer primaries are connected in parallel, and of the two possible ways of doing this only one will work, because of phase considerations. The simplest means of determining the right way is to leave the &volt transformer leads unsoldered when building the relay, simvlv . " twistine them in wlace. After comwletion the relay is tested, and if it does not work these leads are reversed. They may then be soldered and taped.
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The circuit shown was designed for use with a Cenco mercury thermoregulator, which has a metal mercury reservoir. This means that the regulator is necessarily grounded, at the point shown by the dotted ground symbol in the figure, so that plugging in the line with the "live" side towards this point would cause a short circuit. To prevent this the lamp L was connected between the lead from the regulator reservoir and the filament transformer center tap. This lamp is a 110-volt, 7-watt pilot light. If it lights when the power is switched on the plug should be reversed. The circled ground symbol indicates merely on which side the grounded side of the power line should be corinected. The resistance of the lamp does not affectthe operation of the relay.
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SWARTHMGRE COLLEGE SWARTEMORE, PENNSYLVANIA
