156 ADVANCED ELECTRICITY AND MAGNETISM. 



to the product e'q" according to Art. 84. Therefore we have : 



where, according to Art. 84, F is expressed in joules per centi- 

 meter. The force F is the force with which the two concentrated 

 charges attract or repel each other; it is a repulsion when q' 

 and q" are alike in sign, and it is an attraction when q' and q" 

 are opposite in sign. 



The " electrostatic " unit of charge. The amount of con- 

 centrated charge which will attract or repel an equal amount of 

 concentrated charge with a force of one dyne at a distance of one 

 centimeter is used as a unit of charge by many writers on electrical 

 theory, and it is called the "electrostatic" unit of charge. 



To find the value of the "electrostatic" unit of charge in 

 coulombs substitute q for q f and g" in equation (i); write 

 lo~ 7 for F, 1.131 X io 13 for B and I for d; and solve for q. 

 This gives 



one "electrostatic" unit of charge = 3.33 X io~ 10 coulombs (2) 



94. Electric field in the region between two co-axial metal 

 cylinders. A metal cylinder of radius A is surrounded by a hol- 

 low metal cylinder of which the inside radius is B as shown in 

 Fig. I o i, the space between A and B is filled with a dielectric 

 of which the induct! vity is k, the inside cylinder has + q 

 coulombs of charge pe centimeter of length and the outside 

 cylinder has on its inner surface q coulombs of charge per 

 centimeter of length; and it is required to find the intensity e 

 of the electric field at a point p at a distance of r centimeters 

 from the common axis of the two cylinders. 



Imagine a cylindrical surface of radius r with its axis a'ong 0. 

 The electric field e at the surface of this cylinder is everywhere 

 the same in value and everywhere at right angles to the surface. 

 Let us consider a portion of length / of the two cylinders and 

 of the cylindrical surface of radius r. The area of this portion 



