178 



Professor J. A. Fleming 



[May 21, 



ubat speaking to the carbon transmitter modulates the amplitude of 

 the aerial current, and therefore the amplitude of the radiated waves 

 in accordance with the speech wave form (Fig. 12). 



At the receiving station these electromagnetic waves impinge on 

 the receiving aerial and create in it very feeble alternating currents, 

 which are a copy on a reduced scale of those in the transmitting 

 aerial. These are then amplified by valves in cascade, rectified, and 

 sent through a Bell receiving telephone. The result is that the latter 

 emits sounds which closely imitate the speech sounds made to the 

 distant transmitter (see Fig. 13). 



Although this all seems extremely complicated when described in 

 words, it really works very well in practice. Certain things however 

 are conditions of success. In the first place a generating valve must 

 be a " hard " or high vacuum valve, for if it is " soft " or low vacuum 



Fig. 14. — Generator Valve Set for Wireless Telephony. 



the electron emission from the filament will ionize the residual air 

 and the positive ions will bombard the filament and soon destroy it. 

 Again, we require very high E.M.F. to create a thermionic current 

 of sufficient strength for wireless telephony. This is now obtained 

 by rectifying a high voltage low frequency alternating current by a 

 Fleming two-electrode valve. In practice a small alternator is 

 employed, and by means of step-down transformers a low voltage 

 current is provided for heating the filaments of the three valves (see 

 Fig. 14). A step-up transformer working off the same alternator 

 furnishes a high voltage current which is rectified by a valve, and 

 this gives us the Unidirectional plate voltage required for the gene- 

 rating and control valves (Fig. 13). 



The whole of the appliances are usually contained in a small 

 cabinet, and the Marconi Company have kindly sent one of their 

 J-kilowatt radio-telephone sets for exhibition (Fig. 15). 



Such a transmitter will work over 200 miles, as from Chelmsford 

 to Amsterdam, and transmit speech perfectly. More powerful 



