INSTRUMENTATION Thermistors find special application with instruments that require automatic temperature compensation who are availing them I NVESTIGATORS selves of the many advantages of thermistors as temperature-measuring devices will be interested in the valuable paper by Beck of Canberra, Australia [Beck, Α., J. Sci. Instr. 33, 16 (1956)]. His study is concerned with the stability of thermistors. His measurements are concerned with the glass-enclosed, di rectly heated bead-type, the resistance of which is measured by an a.c. bridge. Careful measurements over periods as long as 17 months indicate a progressive drift with age, but the author feels that an accuracy of 0.02° C. can be attained over a range of 10° C , pro vided the thermistor constants do not change over this period. I t is not stated whether the thermistors are of British or American origin, a point which may be trivial because extensive research on the development and production of thermistors has been done in both coun tries. Also, Beck does not state whether or not the thermistors were subject to constant electrical excitation. This careful work causes us consider able concern because, among the ref erences given by the author, we seem to have claimed a considerably higher de gree of reliability for thermistors than other workers. In the development of micromolecular weight techniques very high precision was attained, but under restricted conditions of range which might be unacceptable in other applications [Muller, R. H., and Stolten, H.
J., ANAL. CHEM. 25,
1103
(1953)].
We had attained short-term stability (30 minutes) of 0.0001 ° C. and a repro ducibility to 0.02° C. over a 6-month period. In our opinion, the most im portant factor is the need for continuous electrical excitation of the thermistor, even when it is not in use as a ther mometer. Interruption of the excitation often required more than 24 hours to regain the original calibration values. The sudden change in characteristics observed by Beck has not been ob served by us, at least with Western Electric Co. thermistors. The advent of a rather complete line of thermistors, bridges, and recording equipment made by the Sargent Co. would seem to provide useful means for going into this important question V O L U M E 2 8, NO. 5, MAY
1956
more extensively. We are at present engaged in studies with such instruments in following the dynamics of the ther mistor. Perhaps it is inherent in this device that it cannot possess the widerange reproducibility of other thermometric devices, or that, conversely, it may require much more attention to operating techniques than other ther mometers. Until these important questions are answered, we still adhere to the view point that the utility of the thermistor resides almost exclusively in its unique electrical properties as a function of temperature rather than its direct use as a thermometer. To be sure, it is most attractive in this role as a con sequence of small heat capacity, rapid response, and small dimensions. As we have mentioned previously, countless instruments require automatic tempera ture compensation and we can no longer follow the chemist's instinct to hang everything in a thermostat. It is pre cisely in this function that the ther mistor is so very useful as a temperature compensator. The nonlinear response is an added advantage, even as it is a detriment in simple thermometry, be cause suitable networks can be fashioned to respond electrically to almost any function of temperature. Beck mentions that " . . . until some method of manufacturing thermistors with completely stable characteristics is devised, it would be unwise to use two or more different thermistors to measure temperature difference in one experi ment, unless a careful calibration of each thermistor was performed both before and after the experiment." It is reasonable to assume and expect that such improvements will be forthcoming in the near future. In the history of the phenomenon much is to be learned. Michael Faraday observed the highly nonlinear response of semiconducting substances, and generations of other ob servers obtained similar results, always differing by orders of magnitude. It was not until Bell Laboratory scientists produced ceramic-like masses, that a fairly high degree of reproducibility could be achieved. Perhaps with all the startling developments in other phases of solid-state physics and such
by Ralph H. Müller
useful techniques as zone purification, the way may be open to make the final improvement in stability. At present then, we still have the question—Is the thermistor a thermometry device which requires meticulous attention to operational technique and therefore is more tedious and temperamental than necessary, or is it to be forgotten entirely as a reliable thermometer and relegated to its more versatile functions as a compensator? Beck has performed a most useful service to investigators in warning about the pitfalls. Certainly the thermistor is not a simple device. Rotary Variable Differential Transformer For a long time the linear variable differential transformer has been a useful transducer for measuring small displacements and it has found numerous applications in measuring and controlling systems. It consists of a primary coil excited by a.c. and two symmetrically disposed secondaries on either side of it, which are connected in series opposition. An iron core can be moved along the axis of the three coils and if it is located symmetrically with respect to the three coils, the output signal will be zero. Small displacements in either direction will result in a linear output signal which is phase-sensitive— i.e., it indicates the direction of motion. A small restoring force acts upon the movable core, but this can be reduced to negligible proportions by using higher excitation frequencies. This versatile device is now available in rotary form, in which the same principle can be used to detect and measure angular displacement. Schaevitz En41A
INSTRUMENTATION
instrument abstracts
Cary
Applied Physics Corporation /Pasadena /California At R i k e r
Laboratories
Cary Model 14 Spectrophotometer leads to discovery of new alkaloid in Rauwolfia series C. Howard Stimmel, Analytical Chemist at Riker Laboratories, says: "With the Cary Model 14 we detected structure in the spectra of crude Reserpine samples which was not revealed with our manual spectrophotometer. Further research led
to the discovery of Canescine, a previously u n i d e n t i f i e d m e m b e r of the Rauwolfia series. Differing only slightly from Reserpine, Canescine has notable therapeutic properties of its own."
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ÎSïWilïIÉÈï^ Investigation of unusual features in crude Reserpine absorption spectra obtained by Riker Laboratories with the Cary Model 14 led to the discovery of the important new alkaloid Canescine (left), a relative of Reserpine (right). Riker chemists particularly appreciate the speed and accuracy of the Model 14, according to Stimmel. He says: " O u r reasons for buying the Cary Model 14 were two-fold. One, the automatic scanning feature enables us to get more spectra in a given time; and two, we get more information from the spectra because of
Riker Laboratories, an ethical pharmaceutical specialties h o u s e with main offices in Los Angeles, California, is primarily engaged in producing hypertensive agents, including alkaloids in pure and mixed form.
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Cary
the instrument's greater inherent accur a c y . O u r l a b o r a t o r i e s a r e using t h e Model 14 eight hours per day, five days per week, for both production control and research. Since purchasing the Model 14, we have been able to quadruple our output of spectra." "Before," Stimmel continues, "we were selective as to what we analyzed because of time limitations. Our research department now sends through anything they are even vaguely interested in analyzing. We feel the performance of the C a r y Model 14 justifies our reading significance into every 'wiggle' of the spectra." Resolving power of the Cary Model 14 is better than 1Â in most of the ultraviolet visible region and better than 3Â in the near-infrared. Stray light is entirely negligible for most applications — less than 0.0001% between 2100A and 1.8 microns, and less than 0.1 ''/< at 1860Â and 2.6 microns. Wavelength scale of the Model 14 is accurate to better than 4Â throughout most of its range: reproducibility is better than 0.5Â. Why don't you consider the advantages of the Model 14's greater resolving power and speed? Write for Bull. AC-13.
instruments:
SPECTROPHOTOMETERS • RAMAN SPECTROGRAPH · ELECTROMETERS VIBRATING REED AMPLIFIERS · INFRARED ANALYZERS · ULTRAVIOLET ANALYZERS t APPLIED PHYSICS CORPORATION / 3 6 2 WEST COLORADO STREET / PASADENA 1 / CALIFORNIA For further information, circle number 42 A on Readers' Service Card, page G9 A
42 A
gineering, l \ 0 . Box 505, Camden 1, N . J., offers the r o t a r y variable differential transformer. I t is a small compact device, resembling a n ordinary radio " p o t e n t i o m e t e r " in appearance, and produces a voltage whose magnitude varies linearly with change in t h e angular position of its shaft. I t is provided with complete magnetic shielding. T h e H V D T uses u p to 50-volt single-phase excitation of t h e p r i m a r y in t h e medium audio-frequency range of 400 cycles to 20 kc. per second. Typical sensitivity is an o u t p u t of 9 m v . per degree of shaft rotation a t 2000 cycles per second, with a n input of 5 volts. T h e linearity is ± 1 % over a 40° t e m p e r a t u r e range and ± 3 % over a 60° variation. T h e i n p u t - o u t p u t characteristic is symmetrical about t h e null point. Presumably there is no significant limit t o t h e resolution in t h e sense t h a t one encounters in a wire-wound potentiometer, because t h e action is inductive. T h e operating torque is extremely low and is equal t o 0.014 inch-ounce. Since there are no rubbing or sliding contacts, this torque rating is a t t r i b u t a b l e to the ball-bearing shaft mounting. T h e shaft can be rotated continuously. T h e moving system also possesses a low moment of inertia, this being 1.8 X 10 ~4 pound inch second 2 . This r o t a r y version of t h e linear differential transformer principle has m a n y advantages. Some earlier applications of t h e LDT—for example, in a u t o m a t i c extensometers—applied t h e o u t p u t signal to a strip chart recorder. I n order to provide a null system of measurement, it was customary t o use t h e cable driving t h e recorder pen to move a micrometer, t h e shaft of which repositioned t h e core of a similar L D T , and t h u s cancel t h e input signal. This precise b u t mechanically complicated scheme can now be replaced by the r o t a r y version of t h e L D T . T h e r e are countless applications in which t h e electrical indication of angular displacement is required and from the n a t u r e of t h e o u t p u t a system becomes inherently susceptible to automatic control.
Effect of Humidity Capacity-Controlled
on Relays
Some m o n t h s ago we mentioned the effect of humidity on capacity-controlled relays, wherein it had been shown t h a t the effects were a t t r i b u t a b l e not to surface linkage or change in permittivity of t h e moist air, b u t to t h e properties of aluminum plate condensers. I t now t u r n s out t h a t if capacitor plates are m a d e of anodized aluminum, t h e thin oxide layer becomes very sensitive to ambient humidity, with a reproducible change in capacitance as a result. ANALYTICAL
CHEMISTRY
"For Scientists Everywhere"
B U R R ELL
INSTRUMENTATION
LABORATORY S H A K E R S Build-Up® Design
Size BT
Although originally investigated as a means of tracking down erratic be havior of capacitative relay controls, it now seems possible to utilize the effect as an alternative means of measuring and recording humidity. Recording
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Ο t) Trademark Registered U.S. Patent Office
Wrist-Action Motion
You control procedures from gentle to violent shaking and repeat any operation, exactly, at another time. Side clamps take various shapes and sizes. Top clamps take bottles and Erlenmeyer flasks. Loads need not be balanced. CAT. N O .
ITEM
PRICE
7 5 - 7 6 5 BT
B u i l d - U p W r i s t - A c t i o n Shaker, Size BT for 8 t o p and 8 side flasks
217.50
Price listed is F.O.B. Pittsburgh, Pa. For 115 volts, 6 0 cycle, one phase. O t h e r v o l t a g e s t o o r d e r . For additional
data, ask for Bulletin
No. 307
ONE BASIC UNIT SERVES MANY DIFFERENT REQUIREMENTS
You Build-Up with the new 8-place flat-top and with side arms for 8, 12 or 16 flasks or bottles. One basic unit adapts to any combination.
BURRELL
CORPORATION
Scientific Apparatus and Laboratory Supplies 2 2 2 3 Fifth Avenue, Pittsburgh 19, Pennsylvania For further information, circle number 44 A-1 on Readers' Service Card, page 69 A
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