184 Professor J. A. Fleming [May 21, 



sent at the rate of 180 per minute. Even then the pressure at the 

 surfaces to be brought in contact is so small that not sufficient local 

 battery current can pass to ring the bell unless a plan devised by me 

 for improving such relay contacts is employed. This consists in 

 sending feeble electric oscillations across carbon-platinum contact 

 surfaces.'"" 



It then remains to say a few words on the methods by which the 

 thermionic valve is employed in the reception of signals made by 

 undamped or continuous waves. This system has for many reasons 

 an increasing importance as compared with the damped or intermit- 

 tent wave system. There are four methods in use for creating 

 undamped waves, viz. : — 



1. By a high frequency alternator, which is only an ordinary 

 alternating current dynamo constructed to give currents which 

 alternate 30,000 to 100,000 times a second instead of 25 to 100. 

 Such machines require very exact construction and are expensive to 

 build. 



2. By a Poulsen arc, which is a large direct current electric arc, 

 formed between a carbon and copper electrode placed in a strong 

 transverse magnetic field and in an atmosphere of hydro-carbon 

 vapour. The arc is shunted by a condenser in series with an 

 inductance in which circuit high frequency oscillations are set up by 

 the arc. 



3. By a system invented by Senator Marconi, called the timed- 

 spark, in which a series of condenser discharges are created in 

 sequence and so timed that they produce trains of electric oscillations, 

 one beginning at the instant the previous one ends and in step so as 

 to produce in effect an undamped oscillation. This system is in 

 operation at the great wireless Marconi station at Carnarvon. 



4. Lastly, we have the method by thermionic valves already 

 described. 



By far the best method of receiving signals by these waves is 

 by the so-called beat-reception. 



If two sets of waves of slightly different wave length are 

 super-imposed, no matter what sort of waves they be, the result is to 

 produce a compound wave with periodically increasing and decreas- 

 ing amplitude. These augmentations are called the beats. 



If a continuous electric wave falls on an aerial it creates on it 

 continuous oscillations. Suppose, then, that we generate also by some 

 local means in the aerial wire undamped oscillations differing in 

 frequency, say by 1000, from the incident waves. The result will be 

 to produce in the aerial electrical beats having a frequency of 

 1000. These act to a receiver just as do damped trains of waves 

 with a train frequency of 1000. They can be rectified and detected 

 by a valve and telephone as already explained. When this method 



* See British Patent, J. A. Fleming, No. 112544 of 1918. 



