INSTRUMENTATION sion is linearly proportional to the weight and can be used to measure the weight. To measure this excursion "on the fly." a small shield is mounted on t h e moving system which contains m a n y equally spaced slots. A beam of light passing through the slots falls on a phototube. T h e o u t p u t pulses are shaped electronically and then counted by a preset counter, where they are added and stored. T h e weight can be read directly from t h e counter or the counter can be used t o a c t u a t e sorting devices. Elaborate extensions of this system are described for providing two reject zones and three classifications of acceptable weight. I t has been used for precise sorting of automotive parts and for grading ammunition. For m a n y years t h e Joly spring balance has been used for t h e precise determination of secular changes in weight. In static performance, it has always required meticulous attention and frequent calibration. T h e dynamic approach, so easily handled electronically, is worth)' of further s t u d y in purely scientific applications.
FASTER... MORE ACCURATE ANALYSIS
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ARL'S new X-ray Scanning Spectrometer
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At last — a completely non-destructive X-ray analytical tool that rivals the best chemical methods for accuracy. At last — procedural simplicity, with analytical speeds comparable to directreading optical emission techniques. The new X-ray Scanning Spectrometer with its curved crystals in reflection achieves low limits of detectability — in the parts per million range for the more common metals. These curved crystals give greater intensities and achieve better resolution than possible with flat crystal instruments. The range of wave lengths between 0.35A and 3.88A is covered. All except the lighter elements, those lying below potassium in the periodic table, can be accurately and easily determined. Ratio recording, as used on all Quantometers, results in greater flexibility and ease of calibration: high accuracy is maintained over the complete analytical range. As an analytical research instrument or a production control tool, the new X-ray Scanning Spectrometer provides the utmost in speed, flexibility, sensitivity, accuracy, and precision. Ask an ARL Field Engineer to assist you in determining its applicability to your problems.
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C A L I F O R N I A LAUSANNE, Switzei
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spectrochemicctl instruments include three complete lines designed for Optical Emission Analysis, X-ray Fluorescence Analysis, a n d Raman Spectrum Analysis.
For further information, circle number 34 A on Readers' Service Card, page 37 A
34 A
Measurement of Low Gas Pressure T h e Alphatron, a device for measuring low gas pressures, was described in this column some nine years ago. I t is based upon t h e specific ionization produced in different gases by alpha particles from a small radium source. A new application of this principle provides an extremely sensitive device for fire detection. I t is mentioned here, because it has interesting possibilities for analytical chemistry. I t is curious t h a t no analytical use has been m a d e of the stopping power of substances for alpha, particles despite the fact t h a t long ago Sir William Bragg showed interesting regularities in this phenomenon. T h e detection system is manufactured by Pyrene-C-O-Two Co., Newark, X . J., and a brief description of it is given in Electronics [28, Xo. 6, 200 (1955)]. An ionization chamber, ordinarily filled with air, is ionized by a small radium source. When combustion products are introduced in t h e chamber, the ionization current decreases. I n the practical form of the detector, two chambers are used, one of which is sealed off, t h e other exposed to the atmosphere in which combustion gases are to be detected. T h e difference in voltage drop across the two chambers is applied to the starter electrode of a cold-cathode discharge tube (presumably an A 0 4 G or similar tube). T h e anode current in this t u b e is sufficient to close a s t u r d y relay and sound an alarm or initiate suitable measures against t h e fire. F o r reliable performance, the gas tube envelope is treated to ensure a surface leakage of greater than 10 12 ohms. ANALYTICAL
CHEMISTRY
