Oscillatory Discharges. 7 



(see above), except in the case of the weight determining the 

 openings of contacts having a very great velocity (15 mm./sec. 

 at the least), and special devices being employed enabling 

 the action of the opening spark to be rendered either negligible 

 or, at the least, constant. 



In addition to what has been said it should be borne in 

 mind that those indirect methods cannot be used in the case 

 of very short periods, they being, at the most, capable of 

 ascertaining with security periods not inferior to some ten- 

 millionths of a second. 



The method of spark-photographs, on the other hand, in 

 addition to its allowing very short periods to be measured, 

 is capable of affording also an idea of the importance of such 

 modifications as the spark produces in the movement of 

 electricity. 



Among the forms of apparatus used with this method 

 (e. g. the classical rotating mirror arrangement, Boys' ro- 

 tating objective, Lodge and Glazebrook's rotating photo- 

 graphical films), the two latter ones do not afford a means of 

 reaching very high angular velocities, in addition to their 

 being inadequate for periods of some millionths of a second. 



The turning-mirror arrangement is, without any doubt, the 

 one enabling even very small periods to be measured, pro- 

 vided all those special precautions enumerated below be taken 

 in effecting them. 



B. Metlioal anal Apparatus used in our Experimental 

 Measurement of the Period. 



7. On account of the reasons above quoted we adopted the 

 rotating-mirror method. 



In this method the necessary condition for exact measure- 

 ments is that the distance between the different luminous 

 intervals composing the image of the discharge may be of 

 sufficient magnitude with respect to the width of those in- 

 tervals. Within certain limits this may be obtained by 

 diminishing the magnitude of the image of the spark, and 

 employing high velocities of this image on the film ; this 

 velocity being given by 



v — 4z7rra, 



r being the distance of the film from the mirror, and a the 

 number of turns effected in a second by the latter. 



For practical use it proves more convenient, as a rule, to 

 take r as great as possible and to chose moderate values of a 

 (from 60 to 200 turns per second). But in order not to com- 

 plicate the apparatus by adding rotating arms (similar to 



