A new type of all-electric balancing network for photometric purposes is described, with which a single photo-tube or electron multiplier can be used to locate rapidly the center of a scale mark or spectrum line. This device is now in use on two automatic measuring engines, and is found to give results more reproducible than those involving setting by eye. It is far simpler to adjust and to use than the balanced-cell method previously described. As the enlarged image of a line passes at uniform speed across a slit behind which the multiplier is placed, the variation in light intensity produces a variable output current. The resulting voltage is applied after amplification to a delay network designed to give uniform delay and attenuation to all signals of frequency between 0 and 200 cycles per second. Input and output of this network are combined in phase which is opposite except for the delay, so that the voltage at any instant is balanced against the voltage produced 13 milliseconds previously. Thus, with proper mechanical delay, densities at points on a plate separated by as little as 20 microns can be compared, and a sharp signal is obtained at a position corresponding to the passage of the peak of a line across the slit. This pulse is used to fire a stroboscopic lamp which records instantaneously the reading of a dial which measures the position of the plate. Plates are usually scanned at 4 mm per second, and the positions of as many as 4000 lines can be recorded to within 1 micron or less in 120 seconds.
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