152 ELEMENTS OF STATIC ELECTRICITY. 



cipal effect is produced on the glass, which gives a brill- 

 iant green light ; the illuminated surface terminating in 

 points near the extremities of the negative electrode. 



The influence of induction on the walls of the tube, 

 as well as the conductivity of the glass, is illustrated in 

 Fig. 48 ; which represents a pear-shaped tube, having 

 for its positive electrode an aluminium cross, >, placed 

 near its broad end; the negative electrode a being 

 cup-shaped as in Fig. 46. This cross is hinged at bot- 

 tom to the platinum terminal; so that, by a movement 

 of the tube, it can easily be brought to a horizontal or 

 a vertical position. 



Fig. 48 Inductive Action of Metal Screen. 



When the charge is passed through the tube, the 

 cross, when vertical, as shown in the cut, exerts a strong 

 inductive influence on the broad end of the tube, to the 

 left; over a space inclosed by lines extending over its 

 edges, from the negative electrode : repelling electricity 

 from this space, and screening it from the action of the 

 negative electrode, which attracts electricity from the 

 other parts of the tube, and from the surrounding air. 



