PICTURES BY TELEPHONE.

Otago Witness, Issue 3870, 15 May 1928, Page 25

PICTURES BY TELEPHONE.

NEW SCIENTIFIC WONDER. FURTHER DEVELOPMENT IN JOURNALISM. (From Our Own Correspondent.) LONDON, March 21. Most of the London newspapers have made arrangements to have installed in their •offices apparatuses by which photographs may be telephoned from long distances. The process of photographic transmission has long eluded scientists except in unsatisfactory forms. Wireless photography is an example of the pioneer work which, though wonderful, fails to achieve completely satisfactory results. But it paved the way for telephotographs. Now perfection has been reached. The reproduction is indistinguishable from the original. Io put into nonotechnical language, a most highly technical matter, it may be said that the photo to be transmitted is affixed to a drum, the size and shape of an ordinary jam jar. Shining on it is an all-essential spot of light. The drum revolves and elevates with the result that the spotlight has passed, in thin spirals, over the whole of the picture which is to be transmitted. The principle is the same as in the old phonograph. except that a spotlight is substituted for the needle of the sound box. Inside the cylinder is a small glass bulb coated with potassium. This is a device known as a photo-electric cell. The light which shines through the negative, and which varies in intensity according to the lights and shades of the picture, always falls on the window of the photoelectric cell. Under the illumination the potassium gives off electrons, which pass out in the form of a current of varying magnitude, which registers the delicate gradations of light and shade in the photograph. At the very moment the apparatus at the transmitting end is set in motion current passes along the wires that controls the motive power of the reception apparatus, so that the two are perfectly synchronised. An unexposed photographic film at the receiving end is mounted on a cylindrical frame similar to that of the' transmitting apparatus, and starts to revolve at precisely the moment transmission begins. A device known as a light wave registers on the film all the variations of light and shade brought over the wires by the electric current. The finished result is an exact duplicate of the original photograph. TWO SYSTEMS. In the system of photo-telegraphy developed by the Bell Telephone Laboratories, which is now in operation over the lines of the American Telephone and Telegraph Company, the translation of electrical signals is accomplished by means of a “ light wave.” The electrical signals are made to open and close a small aperature to a varying amount, and thus control the amount of light passing through it on to the recording film. In the same system the synchronism of the two rotating cylinders is brought about by a synchronising electrical signal sent over the same telephone line. In the new Sicmens-Karolus system a photo-electric bell is also used at the sending end, but the receiving apparatus is quite novel. Use is made of the discovery made many years ago by Kerr that the optical properties of transparent substances can be altered by electric stresses. The electrical signals are amplified by a thermionic valve-amplifier, and cause the transmission of light through a cell of nitro-benzole to vary. The synchronisation between the transmitting and receiving cylinders is carried out

without the aid of a synchronising signal along the line. Two tuning forks °are used, one at each end, which are made to vibrate with the same frequency and are kept at a uniform temperature by means of thermostats. The size of the drums for this system permits the simultaneous transmission of two photographs of a size Tin by sin. The speed with which pictures can be transmitted depends on the qualities of the telephone line. V, ith overhead lines a pisture of 16 square inclies can bo transmitted in about a quarter of a minute, but this time is increased to about four minutes when the transmission is made via an underground cable.