Oscillatory Discliarges. 11 



A suitable screen prevented the light emitted by the 

 opening sparks of the electromagnetic tuning-fork falling on 

 the photographic plate (so as to avoid any ghosts prejudicial 

 to the clearness of images) . 



10. Experiments were carried on in the following way: — 

 The pressure of air in the reservoir having reached 5 to 6 

 atmospheres, the photographic plate was placed in position ; 

 then, after starting the electromagnetic tuning-fork, the 

 compressed air was led into the turbine, the rate of its 

 entering the turbine being regulated by means of a con- 

 venient screw-cock, enabling the velocity to be augmented 

 slowly and regularly until the suitable value was reached ; 

 comparison of the note produced by the movement of the axis 

 with that given by the electromagnetic tuning-fork, afforded a 

 means of ascertaining approximately whether this velocity 

 had been reached. 



As a rule, this was so considerable that the proper sound of 

 the turbine had already exceeded the limit of perceptibility, 

 and the sound of the axis was alone to be heard. The con- 

 stancy of this latter sound, and hence the uniformity of speed, 

 could be stated with certainty from a comparison with the 

 sound produced by the electromagnetic tuning-fork maintained 

 in vibration during the experiments. Those two sounds, in the 

 case of most of our experiments, were nearly in unison. 



After having, in the above manner, ascertained the uni- 

 formity of speed of the turbine, the discharge between the 

 spheres of the spark-gap was produced, and immediately on 

 detecting on the photographic plate the reflected image of 

 the mirror, the shutter of the objective was closed, and the 

 rotating cylinder was started, the axis of the turbine and 

 the point of the tuning-fork tracing the respective curves on 

 it. Accordingly we had but to develop and to fix the film 

 by the usual processes. 



As a rule 6 or 7 photographs per period of oscillation were 

 taken for each explosive distance. 



1.1 . The experiments being finished the velocity of the 



image was easily deduced from that of the mirror and the 



distance between this latter and the plate ; a measure of 



the speed of the mirror was next obtained directly from the 



formula -.,_ ~ , 



"w-_ 517*2 .n 



IN 



n 



n being the number of vibrations included in a certain parr 

 of the curve, and n! the number of lines traced on the cor- 

 responding part of the curve by the hair connected to the axis 

 of the turbine, 517*2 being the number of complete oscillations 

 made by the tuning-fork in a second at 25°. 



