124 Dr. K. Domalip on Alternating 



touched at the point m, the rays issued at right angles to the 

 surface and impinged on the opposite wall at m f . If, on the 

 other hand, the tube were touched at n or p, the rays impinged 

 on the inner surface at n' or p' ', at which points bright fluo- 

 rescence appeared. In all cases it appears that the rays issue 

 at right angles to the elements of the surface touched, and 

 radiate into the vacuum-tube rectilinearly till they strike upon 

 the glass wall, where the well-known fluorescence appears. 

 It is easy to further convince oneself of this normal and rectili- 

 near propagation by touching the tube at other points, or by 

 using for this purpose other vacuum-tubes of different form, 

 but in which the vacuum is sufficiently good. Hence it may 

 be concluded from these experiments that these induced 

 currents radiate in the same way as the direct current of 

 the coil in the same vacuum. 



Induced Currents throw Shadoivs of Bodies in their Path upon 

 the opposite Wall. 



For these experiments the tube with the lateral enlarge- 

 ment, already described, is well suited, through the middle of 

 which an aluminium wire passes, -p. 2 



terminating at m in a narrow glass 

 tube. If the entire distance a b is 

 placed in connexion with the earth, 

 there is formed on the opposite wall v 



a shadow of the wire, which is sur- t 



rounded by vivid fluorescence. This 



formation of shadows may be observed with most of Crookes's 

 radiometers. I have employed for this purpose an ordinary 

 radiometer with a mica fly. The bulb, as is well known, is 

 drawn out in the direction of a vertical diameter into a tube 

 of suitable length, which is then placed in a little wooden 

 stand. This tube was surrounded on the outside with a strip 

 of tinfoil near the bottom, which serves as the one electrode; 

 a second disk-shaped electrode of tinfoil was fastened on the 

 outside of the glass bulb at the point where a horizontal dia- 

 meter is intersected by the glass wall. The two electrodes 

 were either connected with the poles of a RuhmkorfFs coil, or 

 one was connected with the rheomotor and the other was put 

 to earth. In both of these cases the rays which traverse the 

 bulb in the direction of a horizontal diameter fall upon the fly 

 of the radiometer, and a corresponding shadow of the fly is 

 seen upon the opposite wall of the bulb surrounded by green 

 fluorescence. E. Wiedemann* has observed a similar phe- 

 nomenon in the use of a spherical positive electrode contained 

 in a spherical glass bulb : when a point of the exterior wall of 

 * Ann, d. Physik u, Chemie, n. F. vol. ix, p. 160, 



