ELECTRIC CHARGE. THE CONDENSER. 



191 



parallel metal plates, 1 5 centimeters apart, the electromotive force 

 between the plates being 2 5,000 volts? Ans. 1,667 volts P er 

 centimeter. 



130. A large metal ball is placed in the uniform electric field 

 between the plates specified in problem 129 and the ball is acted 

 upon by a force of 2.5 dynes. What is the charge on the ball in 

 coulombs? Ans. 15 x io~ a coulomb. 



131. A spark gauge s is connected with two metal plates AB 

 as shown in Fig. 1 1 6. A sheet of paraffined paper 0.002 of an 

 inch in thickness is placed 



between A and B and the 

 spark gap s is slowly in- 

 creased until the paraffined 

 paper is punctured. The 

 spark gap is then meas- 

 ured and found to be 

 equal to o. 16 centimeter. 

 The radius of the spheres 

 of the spark gauge is 0.25 

 centimeter. Find the value 

 of the electromotive force 

 required to puncture the 

 piece of paper. . Ans. 

 6,950 volts. 



133. What is the inten- 



B 



, 



to electric machine 



Fig. 1 1 V 



sity of the electric field in 



volts per centimeter at a 



distance of 200 centimeters 



from the center of a sphere upon which 0.0002 coulomb of charge 



is uniformly distributed ? Ans. 4,500 volts per centimeter. 



134. Find the maximum charge that can be held on a sphere 

 200 centimeters in diameter in air on the assumption that the 

 electric field intensity at the surface of the sphere cannot exceed 

 22,000 volts per centimeter without breaking the air down elec- 

 trically. Ans. 0.000979 coulomb. 



