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40 D. B. JOY AND E. R. GEIB [J. S. M. P. E. If the carbons are not fed together at such a rate as to maintain the arc length approximately constant, the position of the arc with respect to the mirror, the arc length, and the current will all change at the same time. In other words, the changes illustrated in Figs. 2, 3, and 4 all occur simultaneously, and the result on the screen light is a composite of all the curves. Such a composite curve can be cal- culated for a given set of con- ditions, but actual measure- ments are given for one case in Fig. 5. Fig. 5 shows the change in the total light on the screen, the fluctuation of the light, and the change in the current as the arc is allowed to burn from a 0.245-inch length to a 0.58-inch length, without ad- 3oo| (6o r 1 1 1 justment of the transformer the arc controls. The £500 200 J? loo •1 5^ 20- 80- LlQHT DISTRIBUTION .02 .36 or focal position of the carbon for the 0.245-inch length was 3.84 inches from the reflector, corresponding to position C in Fig. 2. The light on the pro- jection screen decreases rap- idly because of the cumulative effect of the decrease in the current and the change in the position of the carbon with respect to the reflector. Besides the rapid decrease light, the lower current causes unsteadiness of the arc, which is discussed in de- tail in a previous paper. 2 Also, if the carbons are allowed to burn apart and are then suddenly adjusted to a much shorter arc length, the sudden change of current and arc length causes a disturbance in the arc which, of course, is transmitted to the light on the screen. It is therefore essential, both from the standpoint of steadi- ness and total light on the screen, that the arc length be held within .// .20 .29 LENGTH IN INCHES FIG. 5. Light on projection screen vs. arc length; carbons not feeding; no adjust- ment of transformer or arc position: 45 of volts, no load on transformer.