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364
HATCH
October
from an indicating meter. This instrument measures 3 inches in diameter and 16Y2 inches in length, and weighs !3/4 pounds, including the indicating meter. It is so constructed that it can be broken down
into two parts for carrying, and will fit in a 9J/2 X 7 X 372-inch carrying case.
CONSTRUCTION
This radiant-energy measuring device is housed in a standard series II 35-mm projection lens barrel with a 4-inch shade tube added. (Fig. 2.) Tubular extensions are provided on both the large and small ends of the barrel. A pinhole aperture 0.080 inch in diameter is placed in the far end of the small-end extension of the lens barrel and a small aluminum block with a tapered hole is fastened to the outside of this pinhole aperture. The sides of this block are approximately 0.55 inch square, so that the entire block can be inserted into the 35-mm picture aperture to a point where the pinhole aperture will come to rest In the pictureaperture plane. The aluminum block serves as a heat baffle to the aperture plate, while the mass of the pinhole-aperture plate serves as an inertia element to prevent too high a temperature being reached on this element, while the reading is being taken. A 10-mm diameter, 12.8-mm focal length, plano-convex lens is placed at a distance of approximately 0.524 inch from the pinhole aperture and is so adjusted to project an image of the pinhole to a plane 13.5 inches from the aperture.
Fig. 1 — Portable radiant-energy measuring device showing lens barrel and indicating meter.