the Velocity of Electric Waves. 



219 



the spark in the secondary circuit, and they showed im- 

 mediately the character of the complex wave-formation. The 

 secondary circuit could and did oscillate in three different 

 ways, and the ratios of the periods were those of the notes in 

 an open organ-pipe, namely 1:2:3. Usually the lowest or 

 fundamental oscillation together with one of the overtones 

 was present ; but several sparks were noticed that furnished 

 unmistakable evidence of the simultaneous existence of all 

 three. We have observed in a circuit 10,000 centim. long 

 the same peculiarities of oscillation, excited by a primary 

 circuit that, judging from its dimensions, could not have 

 been in resonance with the secondary. It was evident that 

 the oscillation having a node between the points marked 

 E and F (fig. 3) is that whose period is one third of the 

 fundamental. 



A number of measurements of this period have been made, 

 and from these values the velocity of the waves has been 

 calculated. The results appear in the following table. As 

 an average of five measurements of the wave-length, none of 

 which differed from the mean by more than 20 centim., the 

 value 5888 centim. was chosen. The distance from the mirror 

 to the photographic plate in each case except the last was 

 300*1 centim. Each -of the first five values in the second 

 column of the table is an average of thirty measurements of 

 distances ranging in the neighbourhood of 1 centim. 



Number of Revo- 

 lutions of Mirror 

 per second. 



Distance between 



two successive 



points on Plate, 



centim. 



Velocity of Waves, 

 centim. 



71-2 

 70-85 

 70-7 

 71-3 



70-8 



•05608 

 •05600 

 •05532 

 •05637 

 •05611 



2-819x101° 

 2810 Xl0 10 

 2-835x101° 

 2-808x101° 

 2-808 xlO 10 



69-2 



Average ... 

 •05340 



2-816x101° 

 2-988 xlO 10 



The last line in the table contains the results of measure- 

 ments on photographs of the primary spark instead of the 

 secondary. In this case the distance from the mirror to the 

 photographic plate was 311*5 centim. 



These results we published as a preliminary record in the 

 American Journal of Science for April 1895. At that time 

 we did not think, judging from the table as it stood, and 

 from a consideration of the possible errors in the various 

 measurements, that the total error in our determination could 

 have been as great as the difference between the average just 



Q2 



