International projectionist (Jan-Dec 1947)

mitted direct viewing of the image by both eyes, or even by several observers. The apparatus was no longer a "peep show." With the two ends separated and with the flat-cathode glow lamps, motion pictures from a projector driving in synchronism with the discs were successfully reproduced in December, 1925. Further work on film was sidetracked by the development of a method of scanning objects without intermediate photographic amplification. A Spot-Scanning Method This beam, or spot, scanning method was devised by Frank Gray. It consists in directing an intense narrow beam of light on the subject and moving the beam rapidly across and from top to bottom on the field in a pattern traced out by the holes of the scanning disc. By this means the average illumination is reduced in the ratio of spot-to-field size (in our case 2,500 times) so that what would be intolerable as adequate floodlighting becomes almost unnoticeable. but remains equally efficient for scanning purposes. This method, it was found, had been previously proposed but apparently with no realization of one of its major advantages: that it is not restricted in use to flat surfaces, as originally disclosed, but is suitable for objects in the round. With this method, light source and photocell are reversed in their role, and it is the photocells, not the light source, that should be made larger and manipulated in position. Cells in multiple, of a size never before attempted, were thenceforth used in appropriate positions around the scanned object. On March 10, 1926, at the conclusion of ceremonies at Bell Telephone Laboratories commemorating the 50th anniversary of the telephone, Laboratory officials talked over a telephone, with the expressions and movements of the face of the speaker being clearly seen by the distant listener. Had our conception of the problem been satisfied by the production of image dissecting and recovering apparatus, operable from one room to another, we could have designated and announced this apparatus as "television." From the beginning, however, it had been considered a necessary part of our obligation as an enterprise engaged in the transmission of information over great distances, to produce for vision a close parallel to what had been done for voice. It would be television when the laboratory experiment was expanded to cover distances beyond any the eye could reach. Large Audience Viewing Accordingly, consideration was given to the problem of putting the photoelectric signals on practical long-distance communication channels. The frequency range from 15 to 20,000 cycles per second generated by the apparatus had to be put on the transmission medium — wire line or radio — at the proper uniform level, free from phase shift, and deliv ered with the necessary amplification at the receiving end, a problem of the same sort, but of exaggerated scale, as in picture transmission. While this work was under way, attention was turned to a method, outlined early in 1925, of exhibiting the picture to an audience of considerable size — the visual equivalent of a public address system. It employed a long neon tube containing 2,500 separate external electrodes, which was bent bick and forth in 50 rows in such a way that there were 50 electrodes in each row. Signals were distributed by a commutator to each electrode in turn, in synchronism with the sending disc. On the grid, .2 by 2% feet in size, a human face and shoulders were reproduced in the pink glow of the neon gas and of a size and brilliancy sufficient to be seen in a moderate-sized auditorium. With a loudspeaker, it reproduced the voice and sight of the subject before the scanning transmitter. In December. 1926, the characteristics of the line-coupling apparatus had been worked out, the "big screen" was functioning, and it appeared possible at an early date to stage a test of actual transmission of vision to a distance. In a memorandum of January 13, 1927, a program for several demonstrations was outlined. Subsequently, however, decision was made for a single demonstration to feature all the apparatus and both wire and radio transmission, by April, three months away. Washington was selected as the far point for the wire demonstration, and the Laboratories' station at Whippany, N. J., for the radio demonstration. Immediately thereafter, the wire transmission arrangements were undertaken by A. T. & T. Additional loading coils had to be manufactured for the existing open-wire lines, cable connections between the Long Lines terminals and the demonstration centers had to be pro(Continued on page 32) Effect of Electric Shock on Human Body ELECTRIC SHOCK may be divided into two broad classes: those due to current from a continuously operating source, and those due to current from a sudden discharge. First, the current from a continuously operating source will be considered. These sources include power lines, whether A. C. or D. C, transformers, rectified power supplies after the initial contact, etc. The most commonly accepted causes of death from electric current may be divided into three classes. First, the current may cause the heart or the brain to lose its power to react to a stimulus. Second, metabolism in the body may be so accellerated that the blood is deprived of its supply of oxygen, and asphyxiation results. Third, sudden violent muscular contractions may cause fatal hemorrhages in various parts of the body, especially in the brain. Body Reaction to Shock Artificial respiration may be effective as a restorative in the first two instances cited, -and especially in the second. The first two causes result usually from mote or less prolonged passage of current; while the third is usually the result of a sudden heavy current. The path which the current takes determines to a large extent the effect upon the body. A path through the heart is usually by far the most dangerous. The current which may be safely passed through the body depends upon the frequency. Frequencies of from 50 to 150 cycles are the most dangerous. D. C. is considered to be equivalent to A. C. at 350 cycles. The human body readily tolerates the following for con siderable periods of time: 8 ma. at 60 cycles, 30 ma. at 11,000 cycles, 800 ma. at 100,000 cycles, and 3,000 ma. at 1 million cycles. While there is no close agreement by various investigators in the field, some sort of weighing of the available information would lead to the conclusion that 75 ma. of 60-cycle current and 150 ma. of D. C. should be considered as a dividing point between safe and dangerous currents. The resistance which the human body presents to an electric current depends largely upon the skin resistance and hence on the type and area of contact. In general it varies between 5,000 and 100,000 ohms in actual cases, with something of the order of 30,000 ohms being average. D.C. Repels; A.C. Holds D. C. usually kicks the subject away from the circuit; while A. C, especially at frequencies around 60 cycles, causes the subject to cling tighter to the circuit. Prolonged contact with the circuit usually causes more and more current to pass due to the breaking-down of skin resistance. Above 50.000 cycles the muscular contractions disappear. United States Census figures show that about 500 people are killed by lightning each year, and about 750 by electric current. Insurance companies state that 50 per cent of electric accidents are fatal. Danger from condenser charge has not been the subject of very extensive investigation. An early investigator found 400 joules fatal to a one-pound guinea pig, and concluded that the number of joules in the charge were more important than the voltage or initial current. INTERNATIONAL PROJECTIONIST • June 1947 21