EXTRANEOUS FREQUENCIES 



167 



calibrated by a thermophone.* The measuring circuit following the 

 search transmitter was calibrated for each frequency measured by 

 introducing the oscillator current into the search transmitter circuit 

 through the attenuator (ai, Fig. 2). 



The ratio of the pressure of the frequency generated along the tube 

 to the fundamental pressure was measured by the attenuator a^, 

 Fig. 2, at various holes along the tube in which the search transmitter 

 was inserted. If an appreciable fraction of the harmonic generated 

 along the tube is reflected at the end of the tube, the magnitude of 

 the reflected component near the source may be comparable with or 

 larger than the harmonic generated between the source and the point 

 in question. This was found to be the case when several measure- 

 ments were made near the source over a distance covering a wave- 

 length. The measured variation in the total pressure of the harmonic 

 over this distance was ± 2.5 db whereas the variation for a funda- 

 mental of this frequency, as previously stated, was ± 0.3 db. There- 

 fore the measurements close to the receiver may be inaccurate. 



Figure 3 shows the measured pressure ratio of the generated second 

 harmonic to the fundamental along the tube and the theoretical curve 



40 50 60 80 100 200 300 400 500 600 800 



DISTANCE FROM SOURCE IN CENTIMETERS 



Fig. 3 — Magnitude of 2nd harmonic vs. distance from source. 



calculated from equation (17). Each of the three experimental points 



plotted at about 45 cm from the receiver is the average pressure ratio 



for a series of readings taken over a distance of a wave-length along 



the tube. 



The measured and theoretical pressure ratios of the second harmonic 



to the fundamental are shown as a function of frequency and pressure 



in Figs. 4 and 5. 



* L. J. Sivian, "Absolute Calibration of Condenser Transmitter," Bell Svs. Tech. 

 Jour., 10, Jan. (1931). 



