the Velocity of Electric Waves. 



223 



resonance is obtained and the most regular wave-formation 

 is excited, the distances between the first three or four dots 

 are slightly greater than the distances between three or four 

 dots taken farther down the spark. The explanation we offer 

 for this is the following, and it applies as a criticism to all 

 cases in which waves are excited in a circuit by a neighbour- 

 ing circuit possessing a much larger damping factor. The 

 fact that the secondary waves last longer than the primary 

 oscillations means that the last times that the waves travel 

 over the circuit they do so under different end conditions 

 from the first few times. The capacity of the secondary 

 plates is slightly less after the primary spark has stopped 

 than it was before, and therefore the length of the wires 

 equivalent to the secondary plates is slightly less, and it takes 

 a shorter time for the waves to travel along the circuit and 

 back. Hence the observed decrease in the distance between 

 the spark- points and a certain mixing up of the dots, which 

 occurs after the sixth or seventh oscillation (see fig. 2). The 

 sixth dot in the figure, apparently following its predecessor 

 after about half an interval, is not a usual characteristic. In 

 the vast majority of sparks the first few dots are far more 

 powerful than those that follow them, and only occasionally 

 do sparks occur that indicate more than five or six good_ 

 complete oscillations. Hence these first few oscillations have 

 the preponderating influence in fixing the length of the waves 

 as indicated by the bolometer. In examining the sparks, 

 therefore, we measured from the first oscillation as far down 

 the spark as we could without passing over a space where 

 dots were obliterated ; and hence in every case we knew the 

 number of dots between the points from which measurements 

 were taken, and did not have to assume that good oscillations 

 had occurred without affecting the plate. 



The following table, containing the results of our measure- 

 ments with the improved apparatus, explains itself. The 

 distance from the mirror to the photographic plate was 302 

 centim. in each case : — 



Number of 

 Revolutions of 

 Mirror per sec. 



Distance be- 

 tween successive 

 points on Plate, 

 centim. 



Time of 



Oscillation, 



seconds. 



Length 

 of Wave, 

 centim. 



Velocity of 



Wave, 



centim. per sec. 



708 

 73-7 

 75-2 

 695 

 68-9 

 690 

 71-2 



•05028 

 •05247 

 •05536 

 •05002 

 •04900 

 •04974 

 •05075 



1-871 XlO 7 

 1-876 x 10 7 

 1-940 xl 7 

 1-897 XlO 7 

 1-874 XlO 7 

 1-899 XlO 7 

 1-878 XlO 7 



5670 

 5670 

 5670 

 5690 

 5690 

 5690 

 5660 



3-030 XlO 10 

 3022 XlO 10 

 2-923 XlO 10 

 3-000 XlO 10 

 3-036 XlO 10 

 2996 XlO 10 

 3014 XlO 10 



Average Value of Velocity 3-003xl0 10 



