308 ME. N. BOHR ON THE DETERMINATION OF THE 



degree of accuracy. However, four satisfactory tubes were found. That the jets 

 produced by these four tubes executed exceedingly pure vibrations appears also 

 very distinctly from the measurement of the wave-length, which will be mentioned 

 later. 



In the experiments the jet had to be placed so that the two symmetrical planes were 

 respectively horizontal and vertical. This was attained by turning the tube into a 

 position midway between two of the above-mentioned readings. 



The Photographing of the Jet. 



To determine the magnitude of the amplitudes of the waves, magnified photographs 

 of the jet were taken. 



Using nearly monochromatic light, and a special limitation of the illuminating 

 beams, the profile of the jet was brought to appear with very great sharpness on the 

 photographic plates. 



By help of an object-micrometer the diameter of the jet was measured at different 

 places of the plate. The single diameters could in this way, on account of the 

 sharpness of the outline, be measured with a relative exactness of about 0'03 per cent. 

 of their value. 



From the largest and smallest diameter of the jet (2r max and 2r min ) the amplitude 



7, = f 1 "- 5 ^^ and the mean radius a = \ (r max . + ?- mill .) \l-\ (|)*1 [see p. 296 (40)] 



a 7 max. T / min. " L " \ a / J 



were determined. (On account of the decreasing of the amplitudes for r max . and r min 

 respectively the mean of two succeeding r max and the r min lying between were used.) 



The value of the mean radius obtained in this way showed a very great conformity 

 with the value which could be calculated by help of the discharge and the velocity 

 of the jet, measured in the manner described above. 



In order to show this, two experiments (executed with the tubes I and IV) will 

 be mentioned, by which the mean radius of the jet was determined in both ways : 



TUBE I. 



''max. 0-06929 cm., 0'06918 cm. Discharge V = 6'274 cm. 3 /sec. 



r mln . 0-06549 cm. Velocity c = 440'8 cm./sec. 



- = 0-0278, a = 0-06736 cm. a = A/ = 



<* V ifC 



0-06731 cm. 



TUBE IV. 



x. 0-08263 cm., 0'08255 cm. Discharge V = 7'862 cm. 3 /sec. 



to . 0-07777 cm. Velocity c = 390'0 cm./sec. 



- = 0-0301, a = 0-08017 cm. a = A/ = 



a V TTC 



O'OSOll cm. 



