TELEGRAPHY OF PHOTOGRAPHS BAKER. 269 



tector. 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 way 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 ex- 

 tinguished, and another tap of the Morse key causes it to light again. 



Now suppose that the taps of the Morse key were controlled by the 

 lines in a photograph or sketch, and that the light from the lamp 

 were concentrated on a revolving photographic film, and you will see 

 at once how a photograph could be transmitted by wireless teleg- 

 raphy. 



Such a process would be utterly impracticable commercially, but my 

 telectrographic system can be used with success in its place. A line 

 picture prepared in the way already described is attached to the drum 

 of the transmitter, and the intermittent current, which is ordinarily 

 passed into the telephone line, goes into an electromagnet, M in figure 

 ('), which then attracts a soft iron diaphragm attached to brass springs, 

 which are fixed to two rigid supports. Every time current flows 

 through the magnet coils this diaphragm is attracted to it, and the 

 platinum contacts PQ are brought together ; when the current flows, 

 and PQ are in contact, the primary circuit of a transformer is closed, 

 and the secondary having a spark gap and being inductively coupled 

 to the aerial and earth, a signal is transmitted into space. Thus in 

 the wireless transmitter the only difference from ordinary telegraphy 

 lies in the fact that the length of the signals and their distance apart 

 are regulated bj the lines composing the sketch or photograph. 



When working with high voltages in the primary, such as 110, 

 arcing is liable to take place, and hence the distance between P 

 and Q when not attracted must be considerable. This means that 

 the distance between the diaphragm clamps must be short, and 

 the German-silver spring of which the diaphragm is made must 

 be thick, these two conditions making the natural period of vibration 

 very short. I have, however, found that by interposing a mercury 

 motor-interrupter in the primary circuit, arcing is almost entirely 

 avoided, as if an arc be formed the current is interrupted an instant 

 later, and the arcing ceases in consequence. 



The receiving apparatus is very simple, and depends, for short- 

 distance work, upon a coherer cymoscope, the decohering apparatus 

 being of a particular character. Every time an oscillation passes to 

 the antenna, the coherer becomes conductive in the ordinary way, 

 and a relay is actuated ; this relay is usually made to start a hammer 

 vibrating, the hammer hitting the coherer, and thus causing it to 

 lose its conductive power. But a vibrating hammer is useless for the 

 photo-telegraphic receiver, and it is essential to have one strike only 

 on the coherer for each signal detected. 



