127. Sciagraphs. Duration of Exposure Dependent Upon Distances. With the given discharge tube, he obtained sciagraphs at a distance of 3/8 inch from the phosphorescent spot in one second, a vulcanized cover being between; at two ft. distant the time was 150 sec.; at three ft., 450 sec.; the opaque plate being interposed each time. Consequently “Roughly, the duration of exposure may be reckoned as proportional to the square of the distance.”
128. Difference Between X-rays and Light Illustrated by Different Photographic Plates. Time of Exposure. The rapid plate for light gave not the deepest images by X-rays. Several different kinds of small sensitive plates were laid side by side. A sciagraph of a metal bar was taken upon them all simultaneously. In this way, he obtained the result, whereby it would appear preferable to employ the mean rapid plate for the purpose of obtaining sciagraphs. On account of the opacity of platinum, it occured to E. B. Frost, (Sci., N.Y., Mar. 27, ’96,) to try platinum photographic paper of the kind used for portraits, but such paper (intended for long exposures in printing in sunlight) was far too lacking in sensitiveness to produce any effect.
128a. Georges Meslins insured a reduction of time for taking sciagraphs by the deflection of the cathode rays by means of a magnetic field. Comptes Rendus, March 23 and 30, 1896. From trans. by Louis M. Pignolet. The method consists in using a permanent or electro-magnet to create a magnetic field perpendicular to the cathode rays in the tube. By this means, the active fluorescent spot on the tube is condensed, and the intensity of the X-rays generated there is increased. Another advantage is that, when the active part of the tube becomes inactive owing to the formation of a light brown deposit upon it, another part can be used by very slightly altering the position of the magnets. Thus, each time a new part of the tube can be used. The magnetic field must not be uniform but must have a suitable variation to produce the desired concentration of the cathode rays.
A. Imbert and H. Bertin-Sans’ Experiment. (Comptes Rendus, March 23, ’96. (From trans. by L. M. P.) They shortened the time by use of a magnet.
James Chappin’s Experiment. (Comptes Rendus, Mar. 30,’96. (From trans. by L. M. P.)—Claimed priority in having shown publicly, on Feb. 19, a sciagraph of a hand, marked “Photograph obtained by concentration of the cathode rays, by means of a magnetic field.” The increase of the intensity of the X-rays obtained by this means was in the proportion of 8 to 5, as measured by the time of fall of the leaves of a Hurmuzescu electroscope.
Prof. Trowbridge, of Harvard University, in a lecture, gave an interesting review (Western Elect., Feb. 29, ’96) of the length of time required in the early days of photography. Improvements are being made whereby the duration required in sciagraphy becomes less and less. In 1827, by heliography, 6 hours’ exposure was necessary; in 1839, by daguerreotype, 30 minutes; in 1841, by calotype, 3 minutes; in 1851, by collodion, 10 seconds; in 1864, by collodion, 5 seconds; in 1878, by gelatine, 1 second. The author remembers the photographs for use in the Edison kinetoscope were taken at the rate of 20 per second. The focus tube brings the time of exposure in behalf of X-rays down to a matter of seconds instead of minutes. For an admirable review of authorities, facts and theories relating to the causes of the darkening of photographic plates by light, see Cottier, in Elect. World, N.Y., May 23, ’96.
129. Size of Discharge Tube to Employ for Given Apparatus.—A small tube required but a small E. M. F., and therefore should be employed with a small induction coil. The greater the distance of the sensitive plate and the object, considered together, from the discharge tube, the sharper the shadow. In short exposures, the tube should be small and at a short distance.
130. Preventing Puncture at the Phosphorescent Spot.—In experiments where he employed a flat cathode, a very thin pencil of rays of increased power came from the exact centre, and in two or three seconds made the glass red hot at the centre of the phosphorescent spot. Immediately, the atmospheric pressure perforated the bulb. This occurred several times. He stated that “the best remedy is to permit the central ray to strike the glass at a low angle; this greatly increases the area and prevents the trouble.” Edison.
Mr. Ludwig Gutmann furnished a translation of a note by Prof. Walter König, found in Eleck. Zeit. of May 14, ’96, relating to this same subject matter. Recognizing that the sharpness of the outlines is the most important requirement in connection with sciagraphy, and that if the rays start from a large surface the impressed shadows will be uncertain in configuration, and noticing, as Edison and Tesla did, [§ 130], the frequent destruction of the tube at the place where the rays were concentrated to a focus, he placed over the inner surface of the glass, aluminum foil for distributing the heat over a larger area, at the same time causing radiation of X-rays from a single point. The focus tube outweighs this in importance. [§ 91].
131. Electrical Dimensions of Apparatus. The best kind of instruction for the student in reference to equipping a plant is to follow the construction employed by those who have been successful. [§ § 106], [109], [114], [137]. Edison used the usual incandescent-lamp current, voltage at 110 to 120 volts, current being continuous, but not connected directly to the induction coil, there being a bank of eight to twenty 16 candle power incandescent lamps arranged in parallel. The interrupter for the primary consisted of a rotating wheel in appearance like a commutator of a dynamo, and was rotated rapidly by a small electric motor, making about 400 interruptions per second, and so constructed that the circuit was closed twice as long as it was open. A sudden interruption was caused by an air blast playing at the point of make and break, the use of which made that of a condenser needless. [§ 3]. The discharge tube terminals were connected respectively and directly to those of the secondary. Prof. Pupin, Columbia Univ. N.Y. (Lect. N.Y., Acad. Sci., April 6, ’96, and Science, N.Y., April 10, ’96) gave valuable and practical instruction concerning the apparatus, which the author witnessed. “A powerful coil was found indispensable for strong effects and satisfactory work. The vibrating interrupter is too slow and otherwise unsatisfactory, and it was replaced by a rotary interrupter, consisting of a brass pulley, 6 inches in diameter and 1-1/4 inches in thickness. A slab of slate 3/4 inch thick was inserted and the circumference was kept carefully polished. This pulley was mounted on the shaft of a Crocker-Wheeler 1/8 H. P. motor giving 30 revolutions, and, therefore, 60 breaks per second. Two adjustable Marshall condensers of three microfarads each were connected in shunt with the break, and the capacity adjusted carefully until the break-spark was a minimum and gave a sharp cracking sound. Too much capacity will not necessarily increase the sparking, but it will diminish the inductive effect which is noticed immediately in the diminished intensity of the discharge. A powerful coil with a smoothly working rotary interrupter will be found a most satisfactory apparatus in experiments with Röntgen radiance.” [§ 106], [109], [114], [131], [137].