28 A
ANALYTICAL
CHEMISTRY
INSTRUMENTATION, continued
"New" Photoelectric
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Transmission Technique for Kilomegacycle Range
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cutting of two serial sections, giving one section for phase microscopic com parison with its serial in the electron microscope, or vice versa. As an example of performance, the Gettner ultramicrotome will cut 0.1micron sections of w-butyl methacrylate to ± 0 . 0 1 micron with an adequately sharpened knife. This is the tolerance of the instrument and does not include the knife-edge, which has its own tol erances. The tissue embedding should not exceed 6 mm. square. The constant-speed motor maintains constancy in all motions at all times. Operation is from 115-volt 60-cycle supply.
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The analyst is required to maintain at least passing interest in microwaves because extensive analytical applications are definitely forthcoming in the near future. When this region of the spec trum becomes more readily accessible to simple measurement, and when fre quency and available power are more flexibly attainable, we shall have one of the most highly specific means of identification known to physical science. Microwave circuitry is complex and its theory is equally involved. If we consider the evolution of transmission lines, it is interesting to note how these have to be modified as one moves toward higher frequencies. At commercial fre quencies, the radiation losses from a conductor are negligible. In the mega cycle range and higher, increasing diffi culty is experienced in keeping the cur rent in the conductor and in preventing its jumping off into space. A parallel pair, the familiar Lecher wire system, minimizes this difficulty. If one imag ines one wire of this pair to be "wrapped" completely about the other, we have two concentric cylinders, which is a coaxial line. In the region of several thousand megacycles (3000 mc. = 10 cm. waves) it is just as convenient to pull out the central wire, thus leaving a pipe. Such pipes, which may be cylin drical or rectangular, form wave guides. The complete transmission properties of such wave guides are calculable from fundamental electromagnetic theory. Junctions, tees, and other directional transitions are more complex, both in calculation and in fabrication. Prac tically all useful cases have been cal culated and reduced to practice. This
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