WIRELESS SIGNALS 131 



suitable for sending signals by wireless. If the 

 current applied to the primary of the coil B can be 

 interrupted by a Morse key, then by tapping for 

 long and short periods we send out trains of damped 

 waves for long and short periods respectively, 

 which correspond to the dashes and dots used in 

 the Morse code. 



Now in wireless photo -telegraphy our trans- 

 mitting machine clearly takes the part of the 

 Morse key, and just as a sentence of words and 

 letters can be made up of dots and dashes, so a 

 photographic image is constructed of long or 

 short marks in proper sequence on the receiving 

 drum of the telectro graph. 



In a simple receiving circuit we have the aerial 

 antenna A and earth plate E connected with the 

 ends of a coil X placed close to another coil Y of 

 very fine wire, which latter transmits the wireless 

 " oscillation " through a condenser K into the co- 

 herer C. A battery B and relay R are also in series 

 with the coherer, which becomes conductive when 

 the aerial receives an electro-magnetic-wave. The 

 relay is thus actuated, and, by means of a local 

 circuit closed by it, a small electro - magnetic 

 hammer is made to tap the coherer, which then 

 becomes non -conductive again, so that the relay 



contact is unmade. 



t 



An ordinary form of coherer is an exhausted 

 glass tube, in the centre of which are two silver 



K 2 



