INSTRUMENTATION Cathode ray oscilloscope recorder, coulometric analysis integrator, cadmium sulfide photocell, and infrared "light pipes" described r p H B modern cathode-ray oscilloscope J- is an extremely versatile and useful instrument. I t is indispensable in electronic and physical research, but there are many analysts who have found numerous uses for it. If the phenomenon under investigation is recurrent and strictly repetitive then, of course, a perfectly steady trace can be presented on the screen by the proper adjustment of sweep frequency and synchronizing pulses. More often than not, it is desirable to obtain a permanent record of the wave shape. This can be done with tracing on translucent paper and more elegantly by photography. In fact, most oscilloscope cameras now employ the Land-Polaroid camera, with which one can obtain a finished positive or negative print in 1 minute. This would seem to leave little to be desired for convenience, but actually the accuracy may not suffice in all cases. A most ingenious scheme has been described recently, which permits one to record such traces on a more leisurely moving pen and ink recorder. As described by C. H. Hertz and E. Moller of the University of Lund, Sweden [Rev. Sci. Instr. 29, 611 (1958)], simple electronic circuitry permits the rapid but repetitive wave pattern to be read off slowly, but accurately, by a recording potentiometer. I t is not possible to describe the exact procedure without recourse to extended circuit diagrams, but the recording is accomplished by the intermittent connection of a condenser to the vertical deflection plates of the oscilloscope by means of a special gate circuit, the gate being opened only during a very short time at a delay time Δί after the horizontal sweep initiation. In this way, the con denser is charged proportionally to the signal amplitude at the time Δί. If the voltage of this condenser is registered by a pen recorder, an exact copy of the signal displayed by the CRO screen will be obtained if Δί is varied slowly from zero to maximum sweep duration. An excellent and convincing example is given by the authors showing a com plex 1000-cycle pattern as photo graphed, together with a pen and ink record of the same wave pattern drawn at a chart speed of 10 cm. per minute. The accuracy is better than ± 2 % as long as not too high frequencies, very steep slopes, or spikes are recorded.
Although subject to improvement, the present arrangement uses a 5-microsecond grating pulse, and the highest frequency which can be recorded with out too much distortion is 10 kilocycles. Numerous suggestions are included for means of handling sharper transients or spikes, but from the indicated per formance, this device would seem to be very useful. We recall with amusement, a time shortly after World War I I , when a distinguished American chemist de livered a tirade against all cathode-ray tubes. In his opinion, they were some what less reliable and reproducible than a Type Κ potentiometer. We agreed completely, but gently offered the sug gestion that they were not supposed to be; that their versatility lay in other directions and that, if the rest of the electronic circuitry was properly de signed, precision would come from other circuit elements. He even agreed that one could use a galvanometer of un known sensitivity to balance a precise Wheatstone bridge. Times have changed. An oscilloscope, which we acquired recently, indicates everything but Greenwich Time! We spent a a whole day reading about its complete and bewildering features. Every sweep, delay, and deflection is subject to in stant checking with high precision; hence its versatility, which has in creased with newer developments, is augmented by precise calibration fea tures. Integrator for Coulometric Analysis
Research in coulometric analysis re quires integrating devices when the process is carried out at constant poten tial. Still another integrator has been described by A. J. Dyer of Melbourne, Australia [J. Sci. Instr. 35, 240 (1958)]. The instrument can integrate an elec trical signal of varying polarity and operates satisfactorily for currents down to 5 μ&. and for electromotive forces down to 0.6 mv. I t is based upon the integrator described by R. W. Gilbert [Rev. Sci. Instr. 18, 328 (1947)], but has the advantage over the latter that it can integrate signals of varying polarity. The integrating mechanism consists of a permanent-magnet, moving-coil system similar to that used for direct
by Ralph H Müller
current indicating instruments, but without a spring restoring force. I t is readily shown that the total angular movement of the coil is an accurate measure of the required integral. A light pointer attached to the coil can engage contacts at both ends of the range of coil movement, and, by means of an associated circuit, the polarity of the signal supplied to the coil is re versed when such contact is made. At the same time, the progress of integra tion is recorded on a digital counter. The switching action, initiated by the pointer touching either of the two con tacts, is performed by one or the other of two flip-flop or univibrator circuits, which have relays connected in one plate circuit. An unregulated 300-volt plate supply is used, and, as the elec tronic circuitry provides only a switch ing action, the accuracy does not de pend in any way on tube character istics. The author's calibration data show linear response for counts per second vs. current for direct current, d.c. ±30 μ&. a.c. and d.c. ±60 μ&. a.c, and, this be havior regardless of superimposed alter nating current, is accurate in the fre quency range direct current to 10 cycles per second to better than 2%. This in strument was designed primarily for meteorological investigations. I t per mitted the daily integration of solar energy with an error of 1%. Cadmium Sulfide Photoelectric Cell
We quote from the Journal of Sci entific Instruments [35, 269, (1958)] a note on an improved cadmium sulfide photoelectric cell manufactured by Mullard Ltd., Mullard House, Torrington Place, London, W. C. 1. This cell is claimed to be some 20,000 times more sensitive than the conventional photoemissive cell. Even from fairly weak sources of light, it is said to operate large relays directly without need for intermediate amplification and with very low voltage applied to the cell. More specifically, with a cell of 1.8 sq. cm. photocathode area, an illumi nation of 5 foot lamberts from a source at color temperature 1500° K, the cell will produce about 20 ma. of current for an applied voltage of 10 volts. At the same level of illumination, but color temperature of 2700° K, the current is VOL. 30, NO. 10, OCTOBER 1958
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77A
INSTRUMENTATION
Cary Applied
Physics
instrument abstracts Corporation/Pasadena/California
Gary Vibrating Reed Electrometer simplifies C14, H 3 and S 3 5 determinations
about 6 ma. Within the limits of per missible power dissipation, doubling the applied voltage gives a fourfold in crease in current. Since t h e device is nonpolar, it will operate from direct or alternating current devices. F o r "onoff" switching operations, speeds u p t o about 50 cycles per second are possible. T h e m a x i m u m power dissipation is 1 w a t t at 25° C. and 200 mw. a t 75° C. T h e d a r k current is very low; a t 300 volts applied to t h e cell, it does not exceed 25 m a . a t 25° C. T h e spectral response covers the entire visible spec t r u m from 4500 to 8000 A and extends into the near infrared with maximum response in t h e yellow-red. Infrared "Light Pipes"
DETECTS AS LITTLE AS 1 0 1 2 CURIES-The
high sensitivity a n d high precision of t h e Cary Model 31 Electrometer p e r m i t m i n i m u m a m o u n t s of costly "tagged" materials to be used in radio active isotope studies, saving enough to pay for the instrument in a short time .and materially reducing the hazard to Tiving experimental subjects. This greater sensitivity, plus the de velopment of simplified experimental procedures, make the Cary Model 31 particularly valuable in determination of C 1 * and H 3 in biological and chemi cal s a m p l e s . W i t h t h e s e n e w t e c h niques, t h e smaller samples may be used and the use of a precipitate with its inaccuracies and time-consuming preparation is eliminated. Instead, sam ples are directly converted into a gas which can be measured with an ioniza tion chamber and a Cary Model 3 1 . The ionization chamber and vibrating reed electrometer offer the only conve nient accurate m e t h o d of measuring radioactivity of CC>2-air mixtures in flowing systems, such as are encoun tered in in-vivo studies. One research group led by Dr. Bert M. Tolbert has had considerable suc cess in applying these procedures to a wide variety of samples and research problems, including studies of animals. NEW REVIEW PAPER Dr. Tolbert, now at the University of Col orado, has authored a 46-page paper cover ing detailed proce d u r e s f o r Cl* a n d Tritium assays, ionchamber theory, samples and sample preparations, combustion of organic compounds to CCV design and con
struction of ion chambers and meas urement of ion chamber currents and approximate calibration data. Copies of the paper are available from Technical Reports Section, Department of Com merce, Office of Technical Services Washington 25, D.C., for $1.25 each. When requesting a copy, please ask for Bulletin UCRL-3499. OTHER USEFUL APPLICATIONS
Measurement of radioactivity is only one of many applications where t h e Model 31 can be used advantageously. For example, amplification and meas urement of ion currents in mass spec trometry, p H determinations, precise measurements of small charges, cur rents, or voltages from a high imped ance source can all be made faster, simpler, less expensively and far more accurately using the Model 3 1 . BRIEF SPECIFICATIONS OF THE MODEL 3 1 Accuracy—Measurements reproducible to within 1 % . Accuracy limited only by the accuracy o* the recorder, potentiometer, or meter used. Reliability-No grid current, greater freedom from zero drift. Much more rugged than other types of electrometers. Stability-Open circuit input; less than 6 χ 1 0 " coulombs rms short period noise; less than 5 χ 10-" amperes steady drift. Short circuit in put; less than 0.02 mv rms short period noise; less than 0.2 mv per day steady drift. 10 standard ranges —Ranges provided on the Model 31 are 1, 3, 10, 30, 100, 300, 1,000 mv, and 3, 10, 30 volts. The output of the Model 31 will operate a 1 ma recording milliammeter or a standard recording potentiometer. Accessories—Ionization chambers, recorders and various special modifications and accessories are available for all types of electrometer uses.
A new catalog on the Cary Model 31 is available. Write for your copy today to Applied Physics Corporation, 362 West Colorado St., Pasadena 1, California. Ask for Data File A9-108.
For further information, circle number 78 A on Readers' Service Card, page 101 A 78 A
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ANALYTICAL CHEMISTRY
The use of quartz glass or plastic rods as "light pipes" is well known. For m a n y applications, light can be " p i p e d " into inaccessible regions, b e n t around corners, and conducted in a m a n n e r which would require elaborate optics if performed in the conventional manner. A recent paper b y Ohlmann, Richards, and T i n k h a m of Berkeley [J. Opt. Soc. Am. 48, 531 ( 1 9 5 8 ) ] , shows how far infrared radiation can be t r a n s mitted through metal pipes with t r a n s mission factors greater t h a n 5 0 % over distances of several feet. I n addition, condensing cones at the terminal end allow the radiation to be concentrated onto a small detector area for spectro scopic applications. Reflection coefficients for metal sur faces in the far infrared are well known, and a few approximations to the com plete theory enable one to compute o p t i m u m conditions for the design of pipes to permit experimental compari son with theory. Light pipes of brass, copper, aluminum, and silvered glass were tested at 70 and 140 microns wave length. Brass pipes were used as a s t a n d a r d because at 0.43 inch diameter, t h e y were most easily polished. F o r "focusing" on a small detector, cones of 2-inch length were fashioned of brass and of aluminum foil formed on a mandrel. B y using these flush against t h e y 8 - i n c h a p e r t u r e of a Golay de tector, a gain of 4.2 to 4.5 could be ob tained. F o r "steering" a r o u n d corners, the authors found t h a t if a pipe is merely bent—even on a large radius— only about 5 0 % of the radiation is transmitted a r o u n d t h e bend. How ever, a right angle t u r n m a y be m a d e t h a t transmits 9 5 % b y joining two pipes—each cut a t 45°—perpendicu larly, and placing a polished plane metal mirror across the joint so t h a t radiation from one pipe is reflected down the other. T h e authors have used stainless steel pipes with success in a cryostat.
