Lecture ON TELEVISION





GIVEN BY j.l. bAIRD AT LEEDS BRITISH ASSOCIATION MEETING, 1927



I propose to give you a brief outline of the principles and general system used in the demonstrations given during last week. Television can be defined briefly as the transmission of visual images by electricity or more briefly as seeing by electricity.

The “Televisor” can be compared to a form of electric camera in which the lens may be, for example, in Glasgow, and the ground glass screen in London. In television, the person who is to be transmitted sits in front of an apparatus which might be compared to the microphone in broadcasting. Only whereas the microphone transforms sound waves into electric impulses, the television transmitter transforms light into electrical impulses. By means of an optical analysing device the image of the object being transmitted is divided into a great number of little sections like a mosaic and each of the little sections constituting the mosaic is projected in succession upon a light sensitive cell. This cell emits a current directly proportional to the light intensity of the section falling upon it at that instant.


The whole image is arranged to traverse the call in the one-sixteenth part of a second. The undulating current produced by the transversal of the image across the cell is transmitted by wire or by wireless to the receiving station. Here it controls the light from a specially designed glow discharge lamp. This light is caused to traverse a ground glass screen by the action of an optical exploring device similar to and revolving synchronously with the exploring device at the transmitter. The spot of light is bright at the high lights and dim at the shadows and so rapidly does it traverse the screen that the image appears as a whole instantaneously to the eye of the observer.


An interesting phenomenon in connection with television is that the human face or scene being transmitted, is first of all converted into a fluctuating electric current and this fluctuating current can be heard on head phones as sound. Every face has its corresponding sound and it is possible to recognise different faces by their individual sounds. These sounds may be recorded on a gramophone record and if this record is played into a televisor, the original image is reproduced. The general principle is therefore comparatively simple. All that is required is an optical exploring device, a light sensitive cell and a glow discharge lamp. The outstanding difficulties in constructing a successful television machine were the construction of a light sensitive device capable of responding to the infinitesimally small light quantities available and at the immense speed necessary, the difficult in procuring a light source capable of giving sufficiently brilliant illumination without time lag and lastly the difficulty of maintaining two mechanisms in synchronism.


The photo electric cell is practically instantaneous in its action, it is however extremely insensitive and requires a very intense light to produce a practical amount of electrical energy. The selenium cell, while it yields an enormously greater current than the photo-electric cell, is inert, that is to say, it takes time for the current to rise to its full value upon exposure to light. In overcoming those difficulties, one of the first methods which suggested itself was the projection of an intense spot of light upon the object being transmitted and traversing the object with this moving spot so that, while the section of the objected affecting the cell was at every instant intensely illuminated, the object itself was not subjected to an intolerably brilliant light.


To accomplish television without the use of an intensely brilliant light required the construction of a light sensitive device capable of re-acting to extremely small light values without time lag and this I found the most difficult part of the problem. I used for experimental work a dummy’s head and for quite a long time I was unable to obtain any response at all from the cell. Then with improved apparatus the dummy began to come through but merely as a black and white effect without half tones or detail. Then one Friday afternoon in the autumn of 1925 I succeeded for the first time in obtaining a true image. The dummy’s head quite suddenly came through, not as a silhouette but with gradations of shading and detail. It was true television at last. I was very excited and ran downstairs to find a human subject. The first person available was the office boy from the office beneath and he rather reluctantly consented to submit himself to the experiment. I left him in front of the transmitter and went into the other room to see what appeared on the receiving screen. This time however nothing appeared on the screen. I was very disappointed but on going back to the transmitting room I found that the boy instead of sitting where I had placed him had been afraid of the intensely brilliant light used, and had retired about a yard away from the transmitter. In the excitement of the moment I gave him half a crown and persuaded him to sit under the light. This time when I went back his head appeared quite clearly upon the little screen. It seemed strange that the first person to be actually transmitted by television should have required a bribe of half a crown for the distinction.


On January 27th, 1926, I demonstrated this apparatus to members of the Royal Institution and the chief point of criticism was the intensely brilliant light necessary. I therefore set to work to reduce this and arranged to get it down to the equivalent lighting of a photographic studio. While engaged on this the thought occurred to me why not use rays outside the visible spectrum? I first attempted to use ultra violet rays. These have an extremely powerful photo-electric effect and also unfortunately very unpleasant physiological effects. The eyes of my sitters soon became affected and the further disadvantage that these rays are to a considerable extent absorbed by glass caused me to make an endeavour to use the rays at the other end of the spectrum the infra red rays. These rays although they have no harmful effect upon the body have only a very minute photo-electric action and their application to television offered considerable practical difficulties. Ultimately, however, these difficulties were overcome with the result that by using the infra red rays in place of light, it was possible to see a person setting in a totally dark room as we have been demonstrating last week.


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A vote of thanks was proposed by Mr. W.G.W. Mitchell, B.Sc., (a member of the British Assoc.) who said: I have listened with great interest to Mr. Baird’s lecture and I have also witnessed a demonstration of Noctovision a few days ago which had the effect of stimulating my interest in an invention of which I had read quite a lot in the press and radio journals. Mr. Baird must have been placed at considerable inconvenience apart from the expense involved in placing his televisor and his own services at the disposal of the members of the British Association, and it is therefore with the greatest of pleasure and confidence that I propose a hearty vote of thanks to Mr. Baird for his Address tonight and I should also like to take this opportunity in view of the wide public interest in television to propose that a Society should be formed forthwith to further the progress of Television and to give a stimulus to this new branch of Science.


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Lt. Col. J. Robert Yelf (a Member of the British Association). I have much pleasure in associating myself with the remarks of Mr. Mitchell and to my mind it speaks well of the courage and confidence Mr. Baird has in his invention to submit it to the critical analysis of the scientific brains of this country. The queues outside the Demonstration Room last week speak for the enormous public interest in the subject of television and Mr. Baird’s invention, and I feel with Mr. Mitchell that no time should be lost in establishing television as a new branch of science and in this connection the suggestion of forming a Society to deal with such a comprehensive subject will I am sure ommend itself to all interested. I therefore have great pleasure in seconding the hearty vote of thanks to Mr. Baird for his interesting lecture tonight and further seconding the proposal that a Society should be formed forthwith to further the progress of television and give stimulus to this new branch of Science. The Chairman put the proposal to the meeting and it was carried unanimously. A further Resolution was proposed by Dr. Tierney, Secretary of the Royal Microscopical Society.





Dr. Charles Tierney seated in front of the Baird Noctovisor apparatus, Leeds, 1927.