186 Professor J. A. Fleming [May 21. 



Sometimes a separate three-electrode valve is used to rectify and 

 detect the heats (see Fig. 19). 



Captain H. J. Round lias, however, invented ingenious methods 

 by which one and the same thermionic valve can be used to generate 

 the beats and at the same time to detect them (see Fig 20). 



There are a large variety of valve connections which have been 

 devised for the same purpose, but time does not permit reference to 

 them." 



We must in the last place glance at the uses of the thermionic 

 valve in connection with ordinary telephony with wires. 



When the rapid fluctuating electric currents flow along a copper 

 telephone line, which are propagated when a speaker at one end of a 

 long hue converses by telephone with an auditor at the other, two 

 effects take place which militate against clear and audible speech 

 transmission. First, the current generally is enfeebled as it flows, 

 and this is called the attenuation. Secondly, the different harmonic 

 constituent currents which go to make up the complex wave form 

 which corresponds to each articulate sound are differently enfeebled. 



The vibrations of high pitch are more enfeebled than those of 

 lower pitch. The first effect reduces the loudness of the speech 

 received, and the second its articulate clearness or quality. The 

 cause of the general enfeeblement is the resistance of the line, which 

 fritters away the energy of the speech electric currents. Until 

 lately the only known method of overcoming it was by putting 

 sufficient copper into the line, but this of course means cost. Thus, 

 for instance, the London to Glasgow telephone trunk line is a double 

 copper conductor of thick wire weighing 1600 lb. per mile run. For 

 the 400 miles or so to Glasgow this requires nearly 300 tons of copper, 

 and the price of that at present is about £30,000 for mere copper. 



This mass of copper is required to give sufficient loudness to the 

 telephonic speech over 400 miles of transmission. 



The thermionic valve is, however, able to make a very large 

 economy in copper. It has already been explained that the three- 

 electrode valve can act as an amplifier. Suppose then that we cut 

 a long telephone line in the middle and insert on one side a 

 transformer, the secondary terminals of which are connected to the grid 

 and filament of a valve, whilst the plate circuit also contains a 

 battery B^ (see Fig. 21) and a transformer of which the secondary 

 circuit is in connection with the continuation of the line. Feeble 

 telephonic currents arriving at the valve would vary the potential of 

 the grid, and this, as just explained, would fluctuate in like manner, 

 but with increased energy, the plate current. The transformer in 

 the plate circuit would then re-transmit the speech current, but with 

 exalted amplitude. 



* The reader may be referred for additional information to a treatise on 

 "The Thermionic Valve in Kadiotelegraphy and Telephony," by -J. A. 

 Fleming, published by The Wireless Press, Ltd. 



