278 PROGRESS TN WIRELESS TELEGRAPHY. 



which are hiid with their siniihir poles together on the coils, their 

 position on the coils being changed until the best results are obtained. 



In the United States, De Forest and Fessenden have also devised 

 autocoherers, which are used in their respective wireless systems. The 

 De Forest autocoherer, which has been termed a "responder," comprises 

 a tube with tin electrodes or plugs running into the bore. Between 

 the inner ends of the electrodes is placed a viscous substance, such as 

 glycerin, in which some lead oxide is suspended as a depolarizer. In 

 shunt circuit Avitli the tube are a small battery and a telephone. Nor- 

 mally the battery sets up an electrolytic action, which tears off small 

 jDarticles of tin from the positive electrode. The«e particles '^ bridge " 

 or '■ thread " over the space between the ends of the plugs, Avith the 

 result that the resistance is much decreased. Incoming electric waves, 

 however, establish electric oscillations in the responder circuit, which 

 disrupt the threads or bridges, whereupon the resistance is at once 

 greatly increased. These variations in the resistance of the circuit 

 are readily indicated in the telephone, and thus when the train of 

 electric waves is broken into dots and dashes of the Morse code mes- 

 sages are easily received. This autocoherer, it will be observed, is of 

 the anticoherer type. 



The Fessenden detector, or " barretter," employs a different princi- 

 ple from either of tlie foregoing. Fessenden takes advantage of the 

 facts that an electric current increases the tem]3erature of a conductor 

 through which it passes, and that an increase of temperature of the 

 conductor increases the electrical resistance of the conductor, and 

 contrariwise. He therefore employs a very thin loop of platinum 

 wire contained in a small glass bulb, the whole so disposed that heat 

 will be quickly conducted from the platinum wire. In the circuit of 

 this loop lie includes a small battery and a telephone receiver, suit- 

 ably connected with the vertical wire. "WHien oscillatory currents are 

 set up in the circuit, rapid variations of the temperature of the 

 platinum loop and corresponding variations in the resistance of the 

 circuit are ])roduced, these in turn affecting the telephone receiver 

 ])ractically as in the instances already given. 



jVIoi'e recently both Fessenden and De Forest have used a detector, 

 termed by the former a " ]i(niid barretter " and b}' the latter an " elec- 

 trolytic receiver," which consists of an exceedingly fine short plati- 

 num wire in a dilute nitric acid solution. The device is outlined in 

 figure 2, in which V is a vessel containing the liquid L ; B is the fine 

 wire, the extent of the immersion of which is adjusted by a suitable 

 screw. The lower wire, ni', enters the liquid from the bottom of V. 

 Its size is not material. Connection with the receiving oscillating 

 circuit is made by means of the wire -w and the wire w'. In the 

 usual shunt circuit from the oscillating circuit there are included the 

 detector, a small battery, and a telephone receiver. 



