Discharges in vacuo. 125 



the bulb is touched with a conductor, a distinct shadow of the 

 electrode surrounded by brilliant green fluorescence is formed 

 on the opposite wall of the bulb. 



Mechanical Action of Induced Currents. 



Crookes, as is well known, has discovered a new mechanical 

 action of the rays issuing from the negative electrode, and 

 has succeeded in setting in motion a little wheel with vanes 

 upon an inclined plane by their action. The question pre- 

 sented itself whether these induced currents would be suffi- 

 ciently powerful to produce a similar mechanical action. For 

 this purpose a vacuum-tube made by Gotze was employed. 

 This tube, whose length was 18 centims. and breadth 4 cen- 

 tims., was furnished with a radiometer in the middle, and had at 

 one end an aluminium electrode so placed that the vanes of the 

 radiometer would be struck by the rays issuing from this elec- 

 trode when made the negative pole, and would thus be set in 

 motion. The second electrode at the other end of the tube 

 is surrounded with a glass tube extending 2 or 3 centims. 

 beyond the aluminium electrode, and then bent at a right 

 angle towards the side of the vacuum-tube. When this elec- 

 trode is connected with the negative pole of the induction- 

 coil, the rays issuing from it can no longer impinge upon the 

 vanes of the radiometer, which therefore is not set in 

 motion. But in this case the radiometer is easily put into 

 motion in either direction by touching one point or another 

 of the glass wall with a conductor. The rotation is easily 

 explained by the production of induced currents on connect- 

 ing the exterior surface of the glass with the earth. Cur- 

 rents are produced which radiate from the surface correspond- 

 ing to the point touched in straight lines into the vacuum-tube, 

 and thus strike the vanes of the radiometer and put it into 

 motion in a definite direction. If then the tube be touched 

 at another point so that the currents thus produced strike the 

 other half of the radiometer, we have motion in the oppo- 

 site direction produced. 



This mechanical action may also be shown with an ordinary 

 Orookes's radiometer coated at two points of the exterior sur- 

 face with tin-foil, as already described, when the coatings are 

 connected with an induction-coil. 



Action of the Magnet. 



In order to investigate the action of the magnet on these 



currents, the vacuum-tube described above with the aluminium 



cross was employed. Whilst the primary induction-current 



passed through the tube, the induced currents were excited 



