ANALYTICAL
20 A
IMPORTANT ANALYSES
w i t h Taylor Comparators . . . f o r pH a n d phosphate determinations Easy to use—accurate results in as little as two minutes—no fragile single standards to handle—these and many more advantages are yours when you use Taylor Com parators for determining pH, chlo rine or phosphate, and the water analyzer for ammonia, silica, cop per, fluoride, etc. All sets are com plete with base, color standard slides, reagents and accessories. Practically all slides work on same base.
COLOR STANDARDS GUARANTEED! There's no danger of mechanical inaccuracies when you use Taylor Comparators because all Taylor liquid color standards carry an unlimited guarantee against fad ing. Further, each complete set of color standards for any one de termination is mounted in a sturdy plastic slide to practically eliminate the problem of breakage.
SEE YOUR
DEALER
for Taylor Comparators and learn how quickly you can make these vital determinations. W R I T E DIRECT for free handbook describing colorimetric methods a n d Taylor equipment for control of p H , chlorine, phosphate, etc. in 3 4 basic industries.
AND CO. 7302 YORK RD. · BALTIMORE-4, MD.
W.A.TAYLOR
CHEMISTRY
INSTRUMENTATION chanical strain or stress, so that new sensing elements and electromechanical transducers may be developed. Since we are completely free to intermix a great variety of powders with various properties, such as semiconductive fer romagnetic particles, photoelectric or photoconductive powders, and the like, the versatility of polaristors may be greatly enhanced. It is curious, but undoubtedly signifi cant, that so much of promise in mod ern physics is related to the behavior of intentionally impure substances or het erogeneous systems. The analyst is always required, and ready, to specify these impurities. Perhaps, the physi cal chemist can be induced to broaden his horizon and look into these things. For all we know, the future graduate student may no longer have to measure some thermodynamic property of a pure substance to four significant figures in order to become a doctor of philosophy, but may be encouraged to study "im pure" substances. The calculated risk— that he may discover something new— is unlikely to frighten him. Related work on the production and properties of isotropic artificial dielec trics is described by R. W. Corkum [Proc. Inst. Radio Engrs., 40, 574 (1952)]. In these systems, small and very uniform conducting spheres are embedded in an insulator such as Styrafoam in precise geometric spacing. The structural dielectric so produced has most interesting properties. The be havior of these materials is most con veniently studied in the microwave re gion. Detector Systems Most of us have come to take for granted the highly dependable perform ance of modern recording potentiom eters. Among the several well known types, the Brown Electronik, the Leeds and Northrup Speedomax, and the Bristol Dynamaster, all make use of a chopper device or synchronous switch which converts a small d.c. signal into an alternating signal which is more con veniently amplified and applied to the servo restoring system. Much has been written about detector systems, but there is continued study of the problem in the incessant search for better signal-to-noise ratio and better all-around performance. A careful dis cussion of the requirements and design for a direct current null detector has been given by Frederick L. Maltby of
the Bristol Co. It appears as a pre print available from the American In stitute of Electrical Engineers, 33 West 39th St., New York 18, Ν. Υ. One question which Maltby has dealt with in great detail is the elimination or minimization of dissymetry effects in the inverter. For a discrimination ratio of 2000 to 1, the dissymmetry must be 0.5% or less in order to prevent overloading due to quadrature voltage. This is difficult to do with semiresonant reed inverters for the fundamental rea son that symmetry, in this class of in verter, depends upon making contact at a consistent position. Thus, the sym metry is subject to variation with wear on any mechanical bias. One way in which the dissymmetry may be held to such a low value is by timing the action to the zero transitions of the supply line voltage. If the line voltage has only odd harmonics, the zero transition points are evenly spaced. Even harmonics are usually present to the extent of less than 0.2% in most commercial power supplies. If the contact position is reversed at these zero transition points or at points in consistent relation to them, the contact action will be symmetrical. This principle has been applied by moving an armature between two lam inated pole pieces which are polarized by a permanent magnet. The arma ture is energized by alternating current of peak value 10 times that necessary to produce magnetic saturation of the ar mature. The action on the armature is essentially a square wave force which reverses abruptly at the zero current point. The resultant acceleration of the armature tip is about 200 "g" and the range of free travel of the tip is re duced to 0.0005 inch, which serves to fix the reversal point still more defi nitely. It is encouraging to note the elegance and unabated attention which are di rected to these studies. The require ments always exceed that which is cur rently possible and it is likely that we shall witness many new approaches to the ideal modulator. The recording of the diverse phenomena required by the analyst presents unusual demands on a modulator, with various input imped ances, frequency requirements, etc. So far, instrument manufacturers have done their very best to keep pace with these unusual demands.
