224 



NATURE 



[August i8, 19 io 



The dividina 



ance that I feel I may be allowed to digress for a few 

 moments to show one way in which they may be utilised 

 to solve a problem that has- long occupied many investi- 

 gators, viz. the satisfactory measurement of the beam of 

 heterogeneous rays from an X-ray tube. Whenever a new 

 tube is used in radiographic worlc, a different voltage, or 

 different interrupter or coil, the time of exposure for the 

 photographic plate has to be determined anew. The 

 strength of the tube under any conditions can, however, 

 be determined by means of a simple piece of apparatus 

 which I have constructed, the working of which I shall 

 now be able to show you. 



If the X-rays fall on a fluorescent screen of barium 

 platino-cyanide, the screen absorbs them and emits 

 yellowish-green visible rays ; this transformed energy is 

 capable of affecting a very sensitive selenium cell when 

 placed in contact with the screen, the resistance becoming 

 less the greater the fluorescence. You will see here a 

 selenium cell of approximately 395,000 ohms resistance, 

 over which is placed a small fluorescent screen of the 

 same size; the cell is put in series with a battery of 100 

 volts and a tnilliampere meter, the divisions of which may 

 be made to correspond to some arbitrary scale or to the 

 time necessary for the exposure of a given make of photo- 

 graphic plate. 



of thp dial depends on two things : first, 

 the characteristic curve of the 

 selenium cell connecting its 

 resistance with the strength 

 of illumination, the linear 

 distance of the source from 

 the cell being, in this case, 

 the most convenient to 

 employ. 



Second, this characteristic 

 curve must be modified to 

 meet the case of illumination 

 by the rays from the anti- 

 kathode, which do not neces- 

 sarily diminish in their power 

 to make the screen fluoresce 

 as the square of the distance 

 from it. You will see on the 

 screen the characteristic curve 

 of a selected selenium cell for 

 feeble illumination, the maxi- 

 mum being of about the same 

 wave-length as that of the 

 fluorescence, showing the re- 

 lation between resistance and 

 distance separating the source 

 of illumination and the cell, 

 and also the modified curve 

 showing a similar relation 

 between resistance and dis- 

 tance between anti-kathode 

 and cell, with the screen in 

 contact. The portion of the first curve most nearly 

 asymptotic is best to employ for the work, and from the 

 second curve the dial scale of the metre can be easily 

 calibrated. If, now, I vary the height of the X-ray tube 

 from the measuring apparatus, you will see that the metre 

 needle is deflected less as the distance between tube and 

 cell is increased. The actual instrument is provided with 

 a scale divided so as to show comparative times of 

 exposure, and by its use radiographic work can be greatly 

 facilitated. 



It is interesting to note that the effect of the rays on 

 the fluorescent screen, as estimated by the selenium cell, 

 differs less with increasing distance the further the anti- 

 kathode is from it : — 



Distance of ami. 



kathode from 



apparatus 



Inches 



6 





o"33 



0'27 

 0'22 

 0-20 



o-i8 



016 



O'o6 

 o'o5 

 002 



0'02 

 002 



A good deal of time has, I am afraid, been taken up in 

 giving details of apparatus ; but I will now show some of 

 the results that have been obtained in practice. The 

 selenium machines already referred to were operated 

 between Paris, Manchester, and London until the end of 

 the year 1908. The first photograph received (slide) was 

 of King Edward, and was received at the Daily Mirror 

 installation in November, 1907. Several results will now be 

 shown in the lantern, and you will observe that they are 

 all composed of parallel lines, which widen or " thin " 

 according to the density of the picture. These lines corre- 

 spond to the movement of the shutter attached to the 

 strings of the Einthoven galvanometer, which regulates 

 the thickness of the spot of light focussed on the revolving 

 sensitive film. This spot of light traces a spiral line round 

 the film, which, when developed, is laid flat, and the spiral 

 becomes resolved into so many parallel lines. 



Late in igo8 Prof. Korn introduced his telautograph, in 

 which a Caselli transmitter, such as already described for 

 the telectrograph, is used, and a line sketch or half-tone 

 photograph is attached to the drum. The receiver is 

 similar to that used in the selenium machines, a spot of 

 light cast on a revolving sensitive film being shut oft 

 every time current flows through the wire of the galvano- 

 meter and displaces it ; when displaced, the shadow^ of the 

 wire falls over a fine slit placed in front of the film, and 

 so prevents the light from passing through to it. A line 

 sketch transmitted from Paris to London in this way is 

 now shown (Fig. 2). The methods of synchronising the 

 sending and receiving cvlinders is the same as that used in 



NO. 2129, VOL. 84] 



the telectrograph; but Prof. Korn's work was done prior to 

 mine, and his arrangements were therefore copied by me. 

 Similar methods have been adopted for many years, how- 

 ever, in certain systems of ordinary telegraphy. 



There is a great deal of interesting matter connected 

 with the efficiency of the galvanometer-receiving apparatus, 

 and the vast amount of careful work done by Prof. Korn 

 to increase it, which time quite forbids my mentioning ; 

 and I will therefore pass on to the latest phase of photo- 

 telegraphic work — the experiments now being carried out 

 to effect wireless transmissions. 



The wireless apparatus for transmitting sketches, 

 writing, or simple photographic images over distances up 

 to about fifty miles may perhaps be looked upon as rather 

 rudimentary, but I shall be able to show, from actual 

 results, that it is at any rate practicable, and it is certainly 

 more simple than any method based on later wireless 

 researches. 



I will first show you an experiment, for the simplicity 

 of which I must ask your pardon ; but it illustrates so 

 clearly how easy it really is to transmit a photograph bv 

 wireless under ideal conditions. I have here a small 

 electric lamp, coupled up with the local side of a relay 

 and battery, the relay being actuated by means of a 

 coherer detector. At the other side of the platform there 

 is a Morse key, which, when depressed, closes the primary 

 circuit of an induction coil, the secondary being coupled 

 up in the usual w'ay to give oscillations. When I press 

 the key. and thereby send a signal, you see that the lamp 

 at once lights up. If the coherer be tapped, the lamp is 



