i6o 



ADVANCED ELECTRICITY AND MAGNETISM. 



denser is left standing on open (This corresponds to the dis- 

 circuit. The coatings of the charge of the condenser.) The 

 condenser are then again con- end of the tube is then again 

 nected and a second slight clamped fast in what seems 

 discharge is obtained. to be its equilibrium posi- 



tion. (This corresponds to 

 the disconnecting of the wire 

 from the discharged condenser.) 

 After a time the end of the tube 

 is again released and a second 

 slight shortening of the tube 

 takes place. (This corre- 

 sponds to the second discharge 

 which is obtained from the 

 condenser.) 



98. Concentration of electrical stresses by points. 



dielectric 



Fig. 106. 



Fig. 1 06 shows the lines of 

 mechanical stress passing 

 around the bottom of a sharp 

 groove in a bent bar. The 

 stress is very greatly concen- 

 trated near the bottom of the 

 groove, and the groove deepens 

 by the formation of a crack. 

 The stress is then concentrated 



H'l'l'l'l'l'l'l'i'l 1 



Fig. 107. 



Figure 107 shows the lines of 

 electrical stress between a flat 

 plate and a point. The stress 

 is very greatly concentrated 

 near the point and if the vol- 

 tage is great enough a rupture 

 of the dielectric begins at the 

 point. The line of incipient 

 rupture is an electrical con- 



