630 Prof. A. Battelli and Mr. I.. Maori on 



r> 



the insulator of the condenser, but to the great resistance 

 offered by the spark on its disappearing. 



But it is a consequence of Stefan 5 s * theory also, that there 

 should always exist a difference between the initial and 

 final values of the period of the discharge. In fact, 

 according to this theory there is added to the undulatory 

 movement corresponding to Thomson's theory another period 

 capable of modifying materially the period during the first 

 moments of the discharge. 



Whatever may be the reason of the above-mentioned 

 displacement of tbe single-spark components, it is clear that 

 this phenomenon affects to a great extent the accuracy 

 attainable in the experimental determination of the period 

 of oscillation. This accuracy, in fact, depends not only on the 

 constancy of the velocity of rotation of the mirror and of the 

 degree of precision with which this same velocity may be 

 measured at the moment of the photograph being taken ; but it 

 depends equally on the precision with which the distance 

 between the photographic images of two successive ele- 

 mentary sparks may be determined. Now, if these sparks 

 are being displaced on the electrodes, the measured distance 

 does not correspond with the true one. 



We, however, eliminated, at least for a great part, this 

 inconvenience of measurements, as, out of the numerous 

 photographs made for each case and for each explosive 

 distance, we used only those on which the images of the 

 sparks were well denned and regularly distributed. 



In addition, as already stated, when calculating the mean 

 value of the distances between two successive elementary 

 sparks, the first and last sparks were always left out of the 

 calculation, and the effective period was derived from the 

 mean distance between the remaining small sparks, these 

 latter being always very numerous. 



The value given for the experimental period was the mean 

 of those derived from the mean of many photographs for 

 each case and each explosive distance. 



The agreement of these values is noteworthy. In order 

 to give an adequate idea of this we record in Tables VIII. 

 to XI. all the values used for determining the experimental 

 value of T for circuits with well-known self-induction. 



Measurements of the same value of the period made on 

 different days always gave perfectly concordant figures. 



From this fact it may be inferred that, in the values of 

 the period measured by us and recorded in the preceding- 

 tables, a notable accuracy has been reached. 

 * Wied. Ann. xli. p. 421 (1890). 



