January 13, 1910J 



NA TURE 



309 



RECENT WORK IN THE TELEGRAPHIC 

 TRANSMISSION OE PICTURES. 



COMMERCIAL photo-telegraphy may be said to 

 have started in November, 1907, when Prof. 

 Arthur Korn installed three of his selenium instru- 

 ments at the offices of the Lokal Anzeiger, in Berlin, 

 L' Illustration, in Paris, and the Daily Mirror, in 

 London ; towards the end of 190S a further selenium 

 apparatus was installed at Manchester. These early 

 machmes depend on the sensitiveness to light of one 

 modification of the element selenium — seleniurn of 

 the slate-grey form distributed over two platinum 

 coils wound one between the other over a flat rect- 

 angular plate of steatite, being termed a selenium cell. 

 Current was passed from a battery through the cell, 

 on which was cast illumination from a Nernst lamp, 

 the rays of which had first to pass through a revolving 

 transparent photographic film, so that the intensity 

 varied each instant according to the density of the 



Sending 

 Station 



photograph. A second cell was illuminated simul- 

 taneously by suitable means, the two being con- 

 nected up on opposite sides of a Wheatstone bridge, 

 so that the current sent to the receiving machine varied 

 as the difference of the reciprocals of the resistance 

 of the two cells. By combining suitable cells the 

 inertia was largely overcome, and a photographic por- 

 trait could be transmitted in twelve minutes. The 

 current at the receiving station passed into a string 

 galvanometer, and laterally displaced a small shutter 

 attached to the "strings," this movement cutting off 

 more or less of the light projected from a second 

 Nernst lamp on to a sensitive photographic film re- 

 volving synchronously with the 

 transmitting cylinder, and one- 

 quarter its size. 



The disadvantage of this system 

 is that the current transmitted is ^l'/^ (C) 

 so small that when there is much 

 leakage on the line it is difficult 

 to get sufficient movement in the 

 shutter of the galvanometer to 

 give a useable result. The maxi- 

 mum current obtainable at the 

 receiving station is about one milliampere. Prof. 

 Korn's telautograph, which he completed in igoS, 

 was therefore a great advance, as, the resistance of 

 the line and the galvanometer being R and r respec- 

 tively, the current C is proportional to E/R-|-r; hence 

 by increasing the electromotive force, more current — 

 up to 20 miiliamperes — can be obtained, and the in- 

 duction effects from neighbouring lines are very much 

 less pronounced. 



The principle of the telautograph of Prof. Korn is 

 seen in diagram form in Fig. i. Here D is a cylin- 

 drical drum about 10 cm. diameter and i2'5 cm. in 

 length, which is revolved at the rate of 30 r.p.m. by 

 a high-speed motor suitably geared down ; S is a 

 steel stylus which is mechanically moved laterally 

 at the rate of about i cm. in 40 seconds, and thus S 

 traces a spiral path over the drum D. To D is at- 

 NO. 2098, VOL. 82] 



tached a piece of copper foil, on which has been 

 drawn a sketch or copy of a photograph in some in- 

 sulating ink ; more recently a single line half-tone 

 reproduction of a photograph in fish-glue on metal 

 foil has been transmitted with considerable success. 

 B is a battery of thirty to sixty volts, and the tele- 

 phone line is represented by dotted lines. A condenser 

 is usually shunted across SD to prevent sparking, 

 about one microfarad being necessary. At the receiv- 

 ing station we have a drum D, of ebonite, on which 

 is wrapped a piece of sensitive photographic film or 

 paper, this revolving in a light-tight box, and also 

 moving laterally in corresponding manner to the 

 transmitter. In the front of the box is a lens 

 and small diaphragm, concentrating as a small spot 

 on the film whatever Hght passes through a fine 

 slit ; this slit lies on the optic axis of a condensing lens 

 fixed in front of the Nernst lamp N, the rays of which 

 pass through a hole bored in the pole pieces of a 

 strong electromagnet M, absorbing about 

 100 watts. A fine flat silver wire is 

 stretched across the magnetic field, as 

 shown in the figure, through which the 

 current received passes ; the shadow of this 

 wire, when no current passes, i.e. when 

 the stylus S is separated from the metal on 

 D by an insulating line of the picture, just 

 covers the slit, but when the stylus is in 

 contact with the metal, and current flows 

 through the circuit, the wire is displaced, 

 and light therefore falls upon the sensitive 

 film. 

 If the period of swing of the galvanometer wire 

 nearly coincides with the period n/t of the currents 

 sent from the transmitter — n being the number cf 

 lines per centimetre and t the time taken for a point 

 on the circumference of the cylinder to travel i centi- 

 metre — there is a great tendency for the wire to 

 vibrate intensely and so not respond exactly to the 

 widths of the lines, which widths constitute in effect 

 the tones in the photograph. Also in transmitting a 

 half-tone photograph with n lines per centimetre, 

 there is always a tendency for the wire to swing with 

 a frequency njt. A very dead-beat effect is, however, 

 obtained by inserting a cell in the line circuit, and 



n 



shunting a regulating resistance on the galvan- 

 ometer, as indicated in Fig. i, and by keeping the 

 moment of inertia of the string to a minimum 

 value. 



I have obtained useful photographic records of the 

 movements of a string galvanometer of very small 

 inertia, in the manner shown in Fig. 2. Here the 

 light from a Nernst lamp N is projected through the 

 hole in a string galvanometer, where it is intercepted 

 by the magnesium shutter F attached to the strings; 

 the shadow of the foil covers a slit S (perpen- 

 dicular to the surface of the paper), and as F moves 

 aside (when current passes through the wires), the 

 effective width of the slit is increased; a revolving 

 sensitive film is actuated by the drums DD, worked 

 by clockwork. The result of transmitting a half-tone 

 photograph, and receiving the same by optical 



