Measurement of Frequency of Electrical Oscillations. 293 



Tims the measurement of the natural frequency of the 

 wave-meter II for a certain position C 2 ' is reduced to 

 the measurement of a length contained between two 

 maxima of sound intensities on the scale S. The number 

 of interruptions of the buzzer is determined from the known 



v 



relation n=~, if v is the velocity of sound in air corrected 

 A, 



for the diameter of the tube used and X the wave-length of 

 the sound in the tube. Choosing the position C 2 " of the 

 condenser to correspond with the first harmonic of the 

 oscillation in position G%, the relation (3) is reduced to 



Vi = su (4) 



For the excitation of the circuit I a buzzer was used, 

 the vibrating part of which consisted of a pair of steel 

 wires q bridged by a soft iron plate t forming the armature 

 of the electromagnet u. By means of a screw the tension 

 of the steel wires could be regulated and the number of 

 interruptions changed from 500 to 1500 per second. 



To check the number of interruptions of the buzzer 

 an interference method was used additionally sometimes. 

 A telephone was connected to the secondary winding of 

 a transformer, supplied with two primary windings — one 

 inserted in the buzzer supply circuit, the other connected 

 to a small alternator, generally used for telephone mea- 

 surements, the speed of which could be exactly measured. 

 By counting the number of beats per second heard in the 

 telephone, the frequency of the buzzer circuit could be 

 calculated. 



The above method was used for calibrating and checking 

 wave-meter standards. Wave-lengths from A, = 3000 m. to 

 \ = 20,000, corresponding to frequencies from 100,000 down 

 to 15,000 per sec, were measured directly by counting the 

 number of maxima s, while shorter waves were then deter- 

 mined by using harmonics of resonating circuits. An 

 accuracy of 0*5 per cent., sufficient for the purpose, was 

 easily obtained. The buzzer could be regulated with such 

 exactness that in continuous work during two hours and 

 more its frequency remained practically unchanged. 



To spare time necessary to find G 2 " by tuning the 

 circuit II to the first harmonic and counting a large 

 number of maxima between two positions of the con- 

 denser C 2 , the second corresponding to a frequency double 

 of that of the first position, the following simplification was 

 adopted in particular cases. 



