INSTRUMENTATION
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866 WILLIS AVENUE ALBERTSON, L. Ι., Ν. Υ. Circle No. 64 A on Readers' Service Card, page 83 A 64 A
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ANALYTICAL CHEMISTRY
roughly in accord with a Newton's law behavior. The rate of rise for the first 10 or 20 seconds is so high that it is indistinguishable from true linearity, for which reason, the author uses the e.m.f. rise in the first 10 seconds. The re sponse cannot be calculated to sufficient precision. It is necessary to calibrate the assembly with at least two speci mens of known thermal conductivity. When the e.m.f. attained in 10 sec onds is plotted against thermal con ductivity, a rapidly rising curve re sults which suggested the true relation ship. A plot of Ε (10 seconds) against the square root of the thermal con ductivity is linear and affords a simple means of calibration. The precision of a single determination is ± 6 % . Lead deviates considerably because, it is thought, of its softness. Surface con ditions affect the results and it has been shown that it is possible to meas ure surface finish in this way. Simi larly, it is possible to measure the thickness of thin foils, especially if they differ appreciably in thermal conduc tivity from the block or anvil on which they rest. Surface coatings can also be measured in this way. The author points out that the actual measurement requires no more than 10 or 20 seconds but as many minutes may be required to reheat the measur ing block in the oven. Modifications have been made to accelerate the re heating of the cooled ball by electrical heating. The system is ideally suited for nondestructive sorting or identifi cation of solid materials. There are several means of rendering some of these operations automatic without destroying the essential sim plicity of the method. We have been studying these and will describe them elsewhere. Thermistor Applications
As we have emphasized repeatedly, thermistors have many uses other than the measurement of temperature. A very stimulating article by A. M. Hardie [J. Sci. Instr. 34, 58 (1957)] shows how series-connected thermistors of the indirectly heated type can be used as a phototube load resistor in order to compensate for the slow drift of illumination from a lamp source and changes in photocell characteris tics. The inherent limitation of com mercially available themistors of hav ing room temperature resistances some what low for this purpose is easily overcome by connecting as many as six in series. By means of feed-back systems, the thermistors can be heated and thus change their resistance. One thus has an automatic gain control for the photoelectric system. Hardie's in
strument was intended for vibration studies and his system succeeded in stabilizing to the extent of 0.03%. For his purpose, the light beam was sym metrically modulated by the vibration under study. The method would seem to have wide application in other pho tometric problems. Better Infrared Detectors
The rapid advances in solid-state physics are paying off in the develop ment of better infrared detectors. Several papers have dealt with the be havior of indium antimonide as a photoconductive detector and of InSb as a p-n junction device behaving as a pho tovoltaic cell. Limit of response is somewhere in the vicinity of 7 microns. Thus, P. W. Goodwin [J. Sci. Instr. 34, 367 (1957)] has studied a single crystal of InSb between room temperature and 90° K. At the higher temperature a typical resistivity is 120 ohms, which rises to 20,000 ohms at 90° K. The relative response increases 1300-fold at the lower temperature with a de crease in wave length of the peak sensi tivity from 6.7 to 5.6 microns. Al though the cell resistance increases on cooling, its value makes the cells still suitable for matching into transistor amplifier circuits. The significant prop erty is the small time constant, which is of 4 X 10~7 second as measured with a pulsed spark source. Related work on InSb is described in another paper by Avery, D. G., Good win, D. W., Rennie, A. E., [J. Sci. Instr. 34, 394 (1957)]. Among idle thoughts for the month, we return to praise of the American telephone system—but in the form of a question. How can it serve the sci entist in the form of audio-telemeter ing? It's being done, of course, in transmitting information from remote pumping stations, etc., but what other useful things could it do for us? As a stunt, we once sent trichromatic data as audio "beeps" to upper Manhattan, where they were relayed over another 'phone line to our laboratory in lower Manhattan and were reconverted to trichromatic beams to reproduce the original color. Will subscribers some day have access to costly computers which can be interrogated in a similar fashion? These are simple things as far as communication scientists and engineers are concerned. It is a ques tion of what sort of information re positories would be useful and desir able, and it is an important question because the widespread and individual use of elaborate computers is, and will continue to be, out of reach for many scientists.
