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A PHYSICAL DENSITOMETER FOR SOUND PROCESSING LABORATORIES*
F. L. EICH**
Summary. — A simple densitometer for use in sound processing laboratories is described, employing the Weston photronic cell and a 50-cp., 12-v. lamp. The cell current is measured by a microammeter, suitably damped. The instrument retains its calibration well, densities can be measured within 0.005, and readings are consistently duplicated.
The applications of light-sensitive devices in industry have been manifold. The development of an efficient photoelectric cell which does not require any external energy supply besides the light incident upon it has increased the number of uses for photoelectric cells. Although prior to the introduction of this type of cell upon the market, it was possible to make a physical densitometer using the other types of cells, nevertheless it always has entailed an additional amount of equipment, which introduced too many variables and made it unsuitable for the commercial field.
The densitometer herein described has simplicity to recommend it without any sacrifice of accuracy. The device has been in constant service for a year at Paramount Laboratory and during that time has held to standard without adjustment (Figs. 1 and 2).
A Weston photronic cell is used as the photosensitive device. The light source consists of a 50-cp., 12volt lamp mounted in a reflector and operated by a storage-battery. A condensing lens throws the light beam onto a plate with a rectangular opening 0.090 by Vie inch, and an opal glass is placed beneath the opening to diffuse the light. A rectangular aperture is preferable because it integrates the density area of the sound-track better than a circular opening. Over this aperture the density to be measured is placed. The photoelectric cell is mounted upon a hinged arm which raises the cell to allow the placing of the film over the light aperture. A shield is placed around the cell so that when it is in the reading position no
* Received August 1, 1934.
** Paramount Productions Inc.. Hollywood, Calif 180