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lamp was set up so as to give the best possible screen, which condition was obtained when the light source was placed at the 2>^'' focus and the distance to the aperture plate was practically 8'', Fig. 4. Under these conditions the lamp which had been used in the preceding test was operated at the same efficiency as before and gave a screen illumination of 616 lumens. When the lamp was removed and the light source at the screen was used for back testing, the lighted area on the test plate was 1 1 by 9 millimeters, a Fig. 5. When the test lamp was moved up to the end of the objective lens there was still an area of 11 by 9 brightly lighted on the test plate, but this was surrounded by a circle b 20 millimeters in diameter. It was noted that the circles which were projected back from the test lamp were somewhat larger than with the plano-convex condenser, showing that the corners of the screen were illuminated by a somewhat greater proportion of the light source. The test also indicated that the corrugated condenser did not utilize as much of the filament area as did the plano-convex condenser.
EFFECT OF MIRROR ON LAMP RATING
In order to determine the effect which the mirror has on the temperature of the lamp filament when the latter is placed at the center of curvature of the reflecting surface, tests were made on both monoplane and biplane lamps. The lamp ratings given in Table i were taken without a mirror focused on the lamp filament. Temperatures were measured both with and without the mirror as shown by the values in the last two columns.
Table 1 — Effect of Mirror on Filament Temperature
H. W. S. W. Temp. With Temp. With
LAMP W. v. Amps. H. CP. P. C. S. CP. P. C. out Mirror Mirror
Monoplane 806 26. 31. 2560 .315 1480 .544 32oo°K 3245°K Biplane 1192 42.3 28.2 3900 .3055 2500 .477 3^75°^ 2,2^$°^
It will be noted that the effect on the temperature of the filament is essentially the same with the monoplane and biplane construction. This is probably due to the fact that in the monoplane construction the coil sections are spaced as closely together as practicable so that the distance between coils is considerably less than the diameter of the coil. The reflected image of the coil, therefore, instead of passing between the coils comes back on the coil to a greater or lesser extent. In the biplane lamp, the reflected image of the coil comes back partially on some portion of the coil and partially on the spaces between so that in the two cases, the temperature conditions are ver}^ nearly the same, the table indicating differences of only 5° with the two coil constructions. If the spaces between the coil sections of the monoplane coil were increased the temperature would probably be somewhat less than indicated, but the screen illumination would also deteriorate because it would show the spaces between the coils.
The effect of a properly focused mirror on increasing the illumination on the screen is indicated in Table 2.
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