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300 D. B. JOY AND A. C. DOWNES [J. S. M. P. E.
pictures and the wider film we are going to need about double the light flux through the optical system that we are now obtaining.
MR. DOWNES: We have made and sold some quantities of carbons for use in a 250 ampere searchlight but as we unfortunately had no machine in which we could burn them, our data are not complete. Calculations from the very meager information available indicate that the light from this arc is at least fifty per cent greater than that from the regular 150 ampere searchlight. This is confirmed by certain tests made by the United States Army.
MR. GRIFFIN: I should like to ask what size the carbons were.
MR. DOWNES: 16 mm.; the same size as the 150 ampere carbons.
MR. MOLE: Some experiments were carried on about eight years ago on the 16 mm. carbon with increased current, and some 25 samples were made, and of these about three had some promise in direct operation. At that time, we found we had an increase of about 50 per cent in illumination at 175 amperes.
MR. BENFORD: With regard to the arrangement shown in Fig. 1, were lenses not used in making this separation? If you used two apertures to get a pinhole image, the image would be very poor. There would not be a clear division between crater and flame light. This shows up too in the curves in the manner in which the scattered light is shown to increase with current. As the current is increased, there is a tendency for the gas from the crater to boil over the sides, and the region surrounding the crater is full of gas. If a sharp image of the crater had been made, it would have been found there is not much increase in the useful flux from the electrode.
MR. DOWNES: Replying to Mr. Benford, we roughly checked the amount of flame light, which was included with what we have called crater light in this set-up, by using both larger and smaller sized openings in screen A and found that with the particular size adopted only a negligible amount of flame light entered the box C in Fig. 1.
In making these measurements no readings were attempted beyond the rating of the carbon, that is, the highest current which would give quiet, steady burning. This seems to me to be the best way to arrive at a rating for a high intensity carbon.
MR. GREENE: Can Mr. Downes tell us at what angle the negative carbon was set?
MR. DOWNES: I think it was 35°.
MR. GRIFFIN : Can Mr. Benford tell us, from the research laboratory check, what the maximum rating is for the 16 mm. carbon?
MR. BENFORD: At 165, it is about right.
MR. GRIFFIN: Did I understand Mr. Downes to say that the 250 ampere carbon was of different construction from the regular 16 mm.?
MR. DOWNES: Entirely different.
MR. GRIFFIN: And you have not arrived at a definite rating?
MR. DOWNES: Yes, sir.
MR. GRIFFIN: Are the carbons available?
MR. GEIB: I think Mr. Benford and Mr. Griffin are referring to different types.
MR. GREENE: Would it be possible or practical to build a 13.6 mm. carbon of the 250 ampere type to burn at more than 125 amperes?