12 



Prof. A. Battelti and Mr. L. Maori on 



In this first part o£ our experiments (where we always made 

 use only of one axis of the turbine) the velocity of the image 

 on the photographic plate was 



d being the distance between the mirror and the plate, this 

 distance being in our case equal to 19*4 cm. 



In order to determine the distances between the images of 

 the different partial sparks, special care and a certain amount 

 of practice was found to be necessary. With our preliminary 

 experiments this determination was effected by making use of 

 a Froment comparator divided in half-millimetres, the vernier 

 giving one-hundredth of a millimetre. On the moving- 

 part of this comparator, carrying the vernier, an eyepiece 

 of small magnifying-power with a cross hair was fixed. 

 Measurements on the plate were made four times, twice 

 in either direction, the mean value being adopted. 



We never found any noticeable differences in the value of 

 the distances comprised between the single sparks corre- 

 sponding to complete periods. 



But as these measurements w T ill often be attended by too 

 great uncertainty towards the end of the discharge owing 

 to the feeble luminosity, we took care to reject those last sparks 

 in our determinations. 



In all cases, however, we made measurements for one 

 and the same discharge as well for such sparks as exhibited a 

 maximum luminosity at the upper electrode., as for those that 

 showed it at the inferior electrode. 



The reading of those distances (NA, NB ; fig. I), as deter- 

 mined directly on the plates, was 

 then reduced to the arc of the 

 circle. The arc « = A'B' will 

 afford the value of T by means 

 of the formula 



T = 



47T. 517*2 .n'.p 



this formula being very suitable 

 for numerical calculations. 



12. We give in the annexed 

 tables some examples relating to 

 the measurements of period of 

 oscillation. D and D / are the 

 distances N B and N A respec- 

 tively of fig. 4, n and ri the numbers of vibrations of the 

 tuning-fork and of the turns of the turbine counted on corre- 

 sponding parts of the respective graphs, p is the number of 

 periods comprised on the measured part of the plate. 



