148 Prof. Q. Majorana : Experimental Demonstration of 



particular form (maximum diameter 4 cm.) bear each three 

 electrodes A, B, E, and are joined try means of strong steel 

 wires with the axle 0. The latter is encircled by two 

 collector-rings K : , K 2 communicating with the electrodes 

 A, B. Two brushes P 1? P 2 bring a continuous current at 

 70 v. A certain quantity of mercury is contained in each 

 tube (28 gr.) and is distributed on A and B when the 

 apparatus is rotating. The electrodes E are provided with 

 light segments o£ aluminium L, which, when the tubes 

 revolve, pass near a metallic arc M, which has the centrum 0. 

 An induction-coil I with its terminals connected as in the 

 figure, placed in action for few seconds, gives discharges, 

 which excite the tubes passing near M. In these the current 

 can be regulated, by interposing the necessary resistance, to 

 between 2 or 3 amperes. I must observe that it is often 

 useless to employ the coil I after having started the rotation. 

 Although in fact the quantity of mercury contained in each 

 tube is absolutely insufficient to establish a continued metallic 

 connexion between A and B, the excitement frequently 

 happens spontaneously by reason of ionization of the re- 

 maining gas, caused by the mechanical shaking during 

 rotation. Besides K x and K 2 , the axle is surrounded by 

 a series of small metallic blocks (not visible in the figure) 

 across which wipes a third brush. This apparatus, like an 

 electric siren, allows a sound to be heard by means of the 

 necessary connexion with a battery and a telephone, by 

 which it is possible to deduce the value of the velocity of the 

 rotation. The light emitted by the tubes is the highest, for 

 constructive reasons, in the tangential direction of the 

 movement. The Michelson interferometer is disposed as in 

 the figure, and on it the light arrives parallel on the mirror 

 S 2 by means of a lens not shown. With the telescope C it is 

 possible to receive a luminous sensation, sufficiently intense 

 in spite of its discontinuity. (20-30 spots per second.) 



Thanks to a sufficient intensity of light, I can use (instead 

 of the above-mentioned experiments with mirrors) a value of 

 / = 232 mm., by which I have observed a maximum of 

 visibility of fringes. In these conditions, and giving to the 

 apparatus a velocity of from 10 to 14 rotations a second, 

 one perceives a displacement of the fringes when the velocity 

 passes from one part to the other. This displacement observed 

 with an eyepiece with micrometer has really the direction 

 that is demanded by the principle of constancy of velocity of 

 propagation of light. Let us foresee its value on this basis. 

 During a long series of observations there is an average : 



