PIERCE. — A METHOD OF MEASURING THE INTENSITY OF SOUND. 387 



at the writer's disposal, so it was decided to calibrate the secondary 

 circuit instead of the primary. For this, a 60 cycle alternating voltage 

 could be employed without much error ; for a preliminary experiment 

 had shown that the impedance of the secondary of the transformer was 

 practically negligible in comparison with the resistance of the rectifier, 

 and that the current- voltage characteristic of the rectifier, as far as 

 tests could be made with means at hand, was independent of the 

 frequency. 



The calibration of the secondary circuit was made as follows : The 

 slide wire of a potentiometer was inserted at W in Figure 2, and a source 

 of alternating voltage was applied at AV. The drop of potential in W 



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VOLTS ALTERNATING 



Figure 6. — Current-voltage characteristic of rectifier. 



was known from the resistance of W and the readings of an alternat- 

 ing current ammeter at I. The alternating voltage in W was varied 

 by varying the resistance of W, and the corresponding direct current 

 in the galvanometer was read. These values are plotted in Figure 6. 



If now we replace the current values in Figure 5 by the correspond- 

 ing voltage values in the secondary of the transformer we obtain Curve 

 2 of Figure 5. This curve is independent of the rectifier, and shows 

 the number of alternating volts at the terminals of the secondary of the 

 transformer of Figure 2 for various positions of the reflecting wall in 

 Figure 4. Except for distortion of the wave when the reflector was 

 too close to the pipe this curve is nearly symmetrical about the line 

 of no reflector. 



