A CATHODE-RAY TUBE ALTERNATING CURRENT BRIDGE DETECTOR for CONDUCTIVITY MEASUREMENTS* FRANK HOVORKA Western Reserve University, Cleveland, Ohio AND
ELWYN E. MENDENHALL Research Division of the Cleveland Clinic Foundation, Cleveland, Ohio
I
N ATTEMPTING to substitute a visual method of detecting the minimum point on an alternating current bridge for the usual set of earphones, Breazeale'
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* From a thesis submitted by Elwyn E. Mendenhall to the Graduate Faculty of Westem Reserve University, May 1938, in partial fuiflment of the requirements for the degree of Master of Arts. 1 BREAZEALE, Rcv. Sci. Inrtrumcntr, 7, 250 (1936).
and Waller2have described units utilizing the 6E5 cathode-ray tube.a Further changes by Garman4 and by Koehletd have resulted in a circuit of increased sensitivitv and simnlicitv. The circuits thus described were * ,
g w ~R C ~A m. ~ ~1, 121 ~ (1937). ,
R C A Receiuing Tube Manual, RC-13, p. 73. GARMAN. R. L.. Rev. Sci. Inrtumeds, 8, 327 (1937). KOEHLER, J. F., ibid.. 8, 450 (1937).
T I , Input trhnsformer T,. Power transformer
100:400,0W ohms
650 volts center tap, 40 milliamperes 6.3 volts, 1 ampere 5 volts, 2 amperes R,,500,000 ohms, 1 watt R2.2200 ohms. 2 milliamoeres Ra;3 megohmd, 1 watt R,, 500,000 ohms, 1 watt R6, RI, 1 megohm, 1 watt Re. 40.000 ohms. 1 milliam~ererheostat Ri; 15;000 ohms; 25 watts Ro, 25,000 ohms, 25 watts C,, 25 microfarads, 25 volts, electrolytic C. 0.05 microfarads. 400 volts c;; 0.1 microfarads, 25 volts Cs,8 microfarads, 500 volts, electrolytic L, 22 henries
in general adapted to inductance and capacitance bridges. When such a detector is needed in a chemical laboratory it is usually in connection with conductivity meas-
urements. For this purpose i t can be further simplied, thus increasing the ease of construction without decreasing the sensitivity. This modified circuit is shown in the diagram.
Because of the large voltage gain of the input transformer the use of a second 657 stage is not necessary for ordinary measurements since a rectified power source can then be used only when the utmost precautions have been taken to avoid hum. Any rectifier hum which is present will tend to open the eye without fuzzing the edges. It is then necessary to reclose the eye by an adjustment of the rheostat RE; an adjustment which slightly decreases the sensitivity of the detector. For this reason it is desirable that any rectifier hum be kept to a minimum. In general practice it seems advantageous that two mountings rather than one be used. In this way the rectifier is not limited to a single purpose but is in a form in which it may find other uses in the laboratory. When two units are used it is of the utmost importance that the leads connecting the units be carefully shielded and grounded. In the use of this bridge detector the rheostat Re is adjusted so that 6E5 grid is fully biased, that is, until the "eye" bas closed down to a crack. The high frequency alternating current is then applied to the bridge and the "eye" opens, due to the fact that the plate current is on the average increased and more bias is obtained across Re. The minimum point is then found moving the slide wire contact until the eye again closes. The sensitivity of this detector will be found to equal that of a good set of earphones. However, if even
greater sensitivity is required it can be obtained by the use of a simple set of lenses for visual magnification. SUMMARY
A simplified cathode-ray alternating current bridge detector is suggested for conductivity measurements in that: (1) It offersa visual means of detecting the minimum point and thus avoids the necessity of isolating the source of alternating current from the rest of the apparatus and of maintaining a quiet room in which measurements can be made.
(2) I t is especially convenient for work which requires a large number of measurements.* (3) It is equally as sensitive as any detector yet described. (4) It offers, because of simplified construction, a cheap, easily constructed unit which serves both as an amplifier and as a detector. *This detector is now being used in a study of the factors involving the solubility of compounds which form renal calculi; a coijperative research between the authors of this paper and Dr. C. C. Higgins and Dr. D. Roy McCullagh of the Cleveland Clinic.
