TELEGKAPHY OF PHOTOGRAPHS BAKEK. 273 



wireless work has been great!}' facilitated by the courteous assistance 

 so readily given by the Marconi Company. 



Tiie general form of the Einthoven galvanometer is well known, 

 and the modified type of it used by Prof. Korn for phototele- 

 graphic purposes has been already shown. If, now, we make the 

 magnetic field very much more intense by building the field magnets 

 heavier, and using a large number of ampere turns in the winding, 

 and also employ a " string," which is very much more elastic than the 

 silver ribbon, the displacement of the string will be correspondingly 

 greater. The silvered quartz fiber used by Duddell for this purpose 

 gives an extremely sensitive instrument, and very appreciable dis- 

 placement is obtained with the current from one diy cell passing 

 through 35 to 90 megohms resistance. 



It is not long since Prof. Fleming explained at this Institution the 

 valve receiver for detecting wireless oscillations; in ordinary wire- 

 less telegraphy, the minute alternating currents are rectified, and 

 sounds are heard in the telephone in circuit owing to small unidi- 

 rectional currents. If these currents be passed through the silvered 

 quartz string of the galvanometer, the string is shifted. If, there- 

 fore, we cause a shadow of the string to lie over a fine slit, any dis- 

 placement will cause the slit to be opened, as it were ; the shadow will 

 be shifted off the slit, and light will be free to pass through it. 

 Oscillations corresponding to the lines in a photograph or sketch 

 could therefore be utilized to cause shifting of the shutter in the 

 manner I have already described for Korn's telautograph, and a 

 sensitive photographic film could be revolved on a drum behind the 

 slit to receive the picture. Such an apparatus is now in course of 

 preparation ; but the amount of light that passes through the slit is 

 extremely small, owing to the fineness of the fiber. Mr. Sanger- 

 Shepherd has therefore attached a minute shutter to the fiber, cross- 

 ing the optic axis; this enables me to use a very much wider slit, 

 and also to adopt the alternative procedure for reception, which you 

 will now see represented in the diagram on the screen. 



For photographic reception, the oscillation is passed into the valve 

 detector, and thence to the quartz fiber AB, which is stretched across 

 the field of the magnets (not shown), the poles of which are bored 

 with a tunnel, through which the beam of light is directed. When 

 the fiber is displaced, light is enabled to pass through a fine slit W, 

 and so act on the photographic film. Where, however, the shutter is 

 attached to the fiber, a much wider slit can be used, and then a pair of 

 narrow compensated selenium cells SS are placed behind the slit W, 

 a positive lens being interposed. When a signal corresponding to 

 a dot in the photograph (i. e., the traversal of a line by the stylus) is 

 received, the fiber shifts, light falls on the cells SS, and their resist- 



97578°— Sit 1910 18 



