Oscillatory Discliarges. 629 



Part IV. Discussion of Results. 



33, In order to have an adequate idea of the importance 

 to be ascribed to the noteworthy agreement observed between 

 the experimental and calculated values o£ the period T, it 

 will be necessary to give an account of the accuracy attained 

 in the determination of the various elements relative to 

 this measurement. 



(a) Experimental Determination of the Period. 



It will be useful here to consider briefly what results may 

 be derived from the numerous photographs obtained. 



First of all there is to be noted that the section of the 

 spark is very large when a considerable amount of elec- 

 tricity is discharged, as in the cases recorded in Tables I., 

 II., VIII. a, IX. a, IX. 6, X. a, and X. /9. 



It next will be worthy of notice that the successive sparks 

 a discharge is made up of do not always pass between the 

 vertices of the spherical calottes forming the electrodes of 

 the spark-gap, but as a rule take various positions on the 

 electrodes, thus lengthening the explosive distance. This 

 phenomenon is clearly seen on many of our photographs, 

 where the aspect of the discharge is that of a strip, whose 

 width, as a rule, increases with time (fig. 2, Plate XV.). 



A behaviour more regular in this respect is noted when 

 the sparks are longer, and when a smaller amount of elec- 

 tricity is being discharged. Then the section of the spark 

 is very small, and the variations of length of the photographic 

 images of the various elementary sparks are less. 



Now if the spark is produced between rather volatile elec- 

 trodes — as, for instance, cadmium electrodes — its behaviour 

 is very regular, as may be seen from photograph No. 3 on 

 Plate XV. If the electrodes consist of platinum or platinum- 

 iridium, the spark as a rule shows a very irregular aspect, 

 as that of photograph No. 2 of the above-mentioned Plate, 

 where noticeable displacements between the single partial 

 sparks are to be observed, each of which, so to speak, has to 

 open itself a discharge- way of its own. 



On this displacement of the elementary sparks may depend 

 the variability of the period of oscillation from one elementary 

 discharge to another, as observed by Trowbridge and Sabine, 

 as well as the fact noted by Lodge and Grlazebrook that the 

 last period of the spark is longer than the rest, this variation 

 being probably due, not to the hysteresis of the air forming 



Phil. Mag. S. 6. Vol. 5. No. 30. June 1903. 2 U 



