kolster: electromagnetic waves 569 



causes mechanical forces to be applied to the vane due to electro- 

 static attractions and repulsions which will set the vane in vi- 

 bration. Since these forces are small, it is necessary that the 

 damping shall be small. In addition to so designing the suspen- 

 sion that there is very little loss of energy in it, it is necessary 

 to keep the instrument in a vacuum. 



The form of the instrument is such that the capacities can be 

 approximately computed. Hence it is possible to develop the 

 mathematical theor}^ of its behavior. This has been done and 

 the conclusions reached have been checked by experiment. 

 The important conclusions are as follows: 



1. The frequency at which maximum deflection is obtained 

 depends upon the potential of the vane. As the potential of 

 the vane is increased, the frequency at which maximum deflection 

 is obtained, is decreased. 



2. The deflection for a given voltage is inversely proportional 

 to the damping. 



3. As the damping is decreased, the tuning becomes sharper. 



4. The power required to give unit deflection when the applied 

 emf is in resonance with the instrument decreases in the same 

 ratio as the damping. 



Experimentally it has been found that the instrument will 

 detect a current as low as 10"^^ ampere. 



RADIOTELEGRAPHY. — A direct reading instrument for measur- 

 ing the logarithmic decrement and wave length of electromagnetic 

 waves.^ Frederick A. Kolster, Bureau of Standards. Com- 

 municated by E. B. Rosa. 



The measurement of the logarithmic decrement at radio 

 stations is of particular importance, since the United States 

 laws governing radio communication specify, among other 

 things, that the logarithmic decrement per complete oscillation 

 in the wave train emitted by the transmitter shall not exceed 

 two-tenths. ^Yhen persistent oscillations of single frequency 

 are emitted from a radio transmitting station much more selective 

 receiving apparatus may be employed with advantage at re- 



1 Detailed paper to appear in the Bulletin of the Bureau of Standards. 



