192 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



135. A very long metal cylinder, 10 centimeters in diameter, 

 is charged and the amount of charge on each centimeter of length 

 of the cylinder is 3 x IO~ 12 coulomb. Find the intensity of the 

 electric field at a point distant 50 centimeters from the axis of the 

 cylinder. Ans. 0.108 volt per centimeter. 



136. Find the maximum charge that can be held on each unit 

 length of the cylinder specified in problem 1 3 5 on the assumption 

 that the electric field intensity at the surface of the cylinder carv- 

 not exceed 22,000 volts per centimeter without breaking down 

 the air. Ans. 62 x io~ 9 coulomb. 



137. A liquid covers a horizontal plane at a uniform depth of 

 10 centimeters. At a point in this plane there is a hole through 

 which the liquid flows at the rate of 1 5 cubic centimeters per 

 second. Find the direction and magnitude of the velocity of the 

 liquid on the plane at a point distant r centimeters from the 

 center of the hole. Ans. 0.2 38 /r centimeters per second. 



138. Two parallel metal plates each one centimeter in diameter 

 are placed in pure distilled water at a distance of 10 centimeters 

 apart and an electromotive force of 100 volts is connected to the 

 two plates. Find the force in dynes with which the plates attract 

 each other, the inductivity of the water being equal to 90. Ans. 

 32x1 0~* dynes. 



139. The entire region throughout a room is a uniform electric 

 field directed vertically upwards and its intensity is 2,000 volts 

 per centimeter, (a) Choosing the floor as the region of zero 

 potential, what is the potential at a point 150 centimeters above 

 the floor ? (fr) What kind of lines are the lines of force, straight 

 or curved, and in what direction ? (c) What kind of surfaces are 

 the surfaces of equipotential, plane or curved, and in what direc- 

 tion do these surfaces lie ? Ans. (a) 300,000 volts, (fr) Straight 

 lines, perpendicular to floor, (c) Plane surfaces, parallel to floor. 



140. Given two parallel metal plates 1 5 centimeters apart to 

 which a io,ooo-volt battery is connected. Imagine a line X 

 drawn straight from plate to plate. Choose the negatively 



