September 23, 1920] 



NATURE 



115 



The Electrical Transmission 01 Pictures. 

 By Philip R. Coursey. 



THE art of the transmission of a picture or 

 drawing by electrical means from a given 

 place, so that it can be reproduced in another, 

 may, speaking generally, be said to date from the 

 perfection of Bain's early chemical telegraph 

 about 1842. In its original form Bain's apparatus 

 was devised for purely telegraphic purposes for the 

 reproduction of the given message at the distant 

 station : with this in view, the message was set up 

 in metal type at the transmitting station, and was 

 connected up with a battery, a number of metallic 

 contact brushes, and a series of line wires between 

 the two stations, so that by moving the brushes 

 across the metallic letters a series of electrical 

 impulses was sent out along the various wires, 

 depending upon the form and arrangement of the 

 letters. At the receiving station a 

 similar series of metallic brushes was 

 passed over the surface of a paper 

 strip, soaked in a solution of potass- 

 ium iodide in starch, with the result , 

 that whenever a brush at the trans- 

 mitter rested upon a metallic part of 

 a letter a current flowed through the 

 corresponding line and through the 

 wet paper strip at the receiver, de- 

 composing the KI solution and giv- 

 ing a blue stain through the reaction 

 of the free iodine with the starch. 

 The resultant marks therefore corre- 

 sponded with the form of the letters 

 arranged at the transmitting station. 



A very slight modification of this simple appa- 

 ratus would enable it to be employed for the repro- 

 duction at a distant place of any sketch or line 

 drawing, and, in fact, also of any picture or photo- 

 graph not having too fine detail. In the case of a 

 photograph, the reproduction at the receiving 

 station would resemble a print from a block of the 

 picture made with a rather coarse screen. This 

 " screen " in the resultant reproduction constitutes 

 the most serious limitation in the quality of the 

 work produced by any of the known methods of 

 photography, and no very obvious way of over- 

 oming this defect is yet apparent. It may be 

 niinimised by accurate mechanical construction of 

 the transmitting and receiving mechanism, and by 

 enlarging the original picture photographically 

 l)efore transmission, so that subsequent reduction 

 of the received reproduction will render the screen 

 less noticeable. 



The apparatus might be arranged somewhat as 

 indicated diagrammatically in Fig. i. A cylin- 

 drical drum A is fitted with a small contact 

 brush B electrically insulated from, but moved 

 l)v, the fine pitch screw S, geared by the wheels 

 \V' to the axis of A. The brush B, therefore, 

 traces out a spiral path on the surface of A when 

 the drum is rotated. At the receiving station, an 

 ' xactly similar drum D and contact C are pro- 



NO. 2656, VOL. 106] 



vided, the two being electrically connected together 

 through the line wires. Each cylinder is provided 

 with a driving mechanism — of clockwork or other 

 convenient motor — and some governing mechanism 

 to maintain a constant speed. Exact synchronism 

 between the two cylinders is essential if an accu- 

 rate reproduction is to be obtained. Uniform 

 driving is therefore necessary unless arrangements 

 are made for synchronisation at the beginning of 

 each revolution by means of a special synchron- 

 ising impulse between the two stations. A phonic 

 wheel, or some similar motor, may be employed 

 with advantage. 



For transmitting the impulses to the line wires 

 with such an arrangement as that outlined above, 

 the original picture may be reproduced in metal by 



Fic. .. 



an ordinary block-making process, and attached 

 to the cylinder A. Contact will therefore be made 

 with the brush B, as the cylinder revolves, at all 

 points which would mark the paper were the same 

 block used for ordinary printing. For the repro- 

 duction at the receiver an electrolytic arrangement 

 similar to that of the original Bain telegraph may 

 be employed, using potassium iodide and starch 

 for the electrolyte, or potassium ferrocyanide with 

 an iron electrode brush C. 



This apparatus in its simple form is evidently 

 suitable only for reproducing sketches or drawings 

 of which ordinary line blocks can be prepared, but 

 with an improved receiver, such, for example, as 

 the photographic one described below, a "half- 

 tone " block made from a photograph could evi- 

 dently be used, if the screen were not too fine. 

 For satisfactory reproduction the travel of the 

 brush B per revolution of \ should be equal to the 

 distance between the screen lines. 



The repeating relay R may be placed when 

 required between the transmitter and the lines to 

 enable smaller control currents to be employed. A 

 similar relay may be placed at the receiving end, 

 if the incoming current is too weak. 



As an alternative to making a metal reproduc- 

 tion of the original picture for transmission by 

 this method, a photograph of the original may be 



