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554 BERKLEY AND MANSBERG November
TYPES OF PATTERNS
We may divide the types of patterns to be photographed into three classes — recurrent, transient, and drifting phenomena. Recurrent phenomena are those which recur reproducibly from cycle to cycle without any interruption or deviation. Transient phenomena are those phenomena which occur once or at specific repetition rates, but which are not in general reproducible. Drifting phenomena are phenomena which have no specific repetition rate or no specific wave shape, or which may be random both as to wave shape, repetition rate or other characteristics, or may be entirely random.1
Let us first consider recurrent phenomena, that is, perfectly reproducible, stationary patterns on cathode-ray tubes. The first problem that one encounters in connection with any pattern is that of obtaining the proper exposure. For an oscillograph having continuous
Fig. 6 — Effect of overexposure and underexposure on line width.
sweeps, the general rule may be stated that at a particular setting of the intensity control, the exposure required with a particular camera will be constant irrespective of the sweep being used or the frequency recorded, and varying inversely with the area which the constant electron-beam intensity covers. For this reason exposure tables in the instruction books and specifications of oscillograph cameras for such cases have been found adequate. It will be noted upon reference to such tables that the exposure called for is constant over a very wide frequency range and is only required to be modified slightly at the very slow writing speeds where the spot is likely to be overexposed. A properly exposed oscillogram should have a quality and clarity capable of supplanting directly the present draftsman's drawings now used in many texts and papers. To demonstrate this, we have used the original negatives so far as. possible as figures. Fig. 6 shows the effect of proper and improper exposure upon a repetitive phenomenon. The recording at the right has been underexposed resulting in a poorly visible trace; the one in the center is properly exposed, and the one at the left is overexposed. It can be seen that the apparent line width of