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VI. The Production and Measurement of Short Continuous: 

 Electromagnetic Waves. By Balth. van der Pol, Jun... 

 Doct. Sc. (Utrecht)"". 



UNTIL recently the only means o£ producing electro- 

 magnetic waves o£ a few metres length was a spark 

 discharge either in air or oil. One of the best known circuits 

 used for this purpose is that due to Blondlot. It has, 

 however, some drawbacks both from experimental and 

 theoretical points of view. The chief difficulty experienced 

 in the experimental work with this circuit is the lack of 

 constancy of the spark. When merely measurements of 

 specific inductive capacity of some material having low 

 conductivity have to be made and a neon tube is used 

 for the indication of resonance, the variations of the wave 

 amplitude due to the unavoidably varying conditions at the 

 spark are not so serious. But when it is desired to measure 

 the high-frequency conductivity of the material under consi- 

 deration, a different method of measuring the wave-intensity 

 has to be adopted, and a higher constancy of wave-amplitude 

 than a spark can give is of great importance. 



The theory of resonance of the Lecher system coupled by 

 means of the first bridge of the Blondlot exciter usually 

 ionores the reaction from the resonator on the oscillator. 

 When, further, the exciting vibrations are supposed to be 

 damped, the theory of the system becomes rather involved. 



By producing the waves, however, with a three-electrode 

 thermionic tube with suitable circuits, it is possible to- 

 eliminate all these difficulties at once. 



The apparatus to be described furnishes us with a system 

 having the following valuable properties: — 



1. The constancy of the current is all that can be 



desired. 



2. The reaction of the resonator on the vibrator can be 



made negligibly small, while the sensitiveness of the 

 indicating instrument can very simply be varied 

 within wide limits. 



3. The continuous vibrations in the oscillator render 



resonance measurements exceedingly sharp. 



4. The wave-length can easily be varied and brought 



down to a few metres, which fact makes the method, 

 particularly suitable for laboratory work. 



* Communicated by Sir J. J. Thomson, O.M., P.K.S. 



