THE PHENOMENA OF ELECTROSTATICS. 



239 



vertical distance fallen by the ball under the action of gravity. 

 If D is known and d observed, then the velocity of the cannon 

 ball is given by the equation 



(0 



in which g is the acceleration of gravity. 



Action of the electrical field on a moving charged particle. 

 Consider a charged particle moving upwards through an elec- 

 trical field as shown in Fig. 1 70. Let q be the charge on the 



lead block , 



\ lead block 



lead blo'tk", Head block 



Fig. 170. Fig. 171. 



Magnetic field perpendicular to plane of paper. 



particle in abcoulombs and e the intensity of the electrical field 

 in abvolts per centimeter. Then the force F in dynes pulling 

 on the particle is equal to qe, so that the acceleration of the 

 particle in the direction of F is qejm. It is evident that the 

 particle moves in the same sort of an orbit as a cannon ball, and 

 that the acceleration qejm corresponds to the acceleration of 

 gravity g in the case of a cannon ball. Therefore, using qejm 

 for g in equation (i), we have 



or 



m 



q 



2dm 



We 



(iii) 



