KNUDSEN'S APPARATUS 129 



scratches was thus produced,, which could be printed 

 from like an ordinary photographic negative. 



The synchronising was effected by the smoked 

 glass plate throwing out the stylus circuit at each 

 end of its travel and switching in another circuit 

 which released it and set it free to travel again. 

 The results were crude and streaky in appearance. 



Mr. Knudsen gave some demonstrations in 1909 

 at the Hotel Cecil, and transmitted a picture of 

 the King, by his wireless machines, across the 

 room in which the apparatus was displayed. 



Like most workers who begin their wireless 

 studies on a modest scale, I used a coherer in the 

 early experiments in order to detect the signals 

 transmitted from the sending apparatus. These 

 signals are of course regulated by the lines or dots 

 of which the photographic image is composed, and 

 the lines in a line drawing or half-tone photograph 

 really act as the interruptors of the primary circuit. 



It will be more convenient to describe briefly one 

 or two of the ordinary methods of wireless tele- 

 graphy, as the description of the photo -telegraphic 

 apparatus will then be clearer to those whose work 

 does not carry them into the " realm of wireless." 



If we take an induction coil as shown in Fig. 58 

 and apply a suitable battery to the primary winding, 

 the current of which can be rapidly interrupted by 

 means of an interrupter I, a spark will pass between 

 the terminals P, P of the secondary coil. If now we 

 P.T. K 



