PROBLEMS AND EXERCISES. 419 



a potential on all external points exactly equal to that of the 

 actual negative electrification. (See Art. 250.) 



Ans. The point must be on the line between the outside 

 positive charge and the centre of the sphere and at 

 5 centims. from the surface. 



13. Two large parallel metal plates are charged both 

 positively but unequally, the density at the surface of A being 

 + 6, that at the surface of B being + 3. They are placed 2 

 centimetres apart. Find the force with which a + unit of 

 electricity is urged from A towards B. Find also the work 

 done by a + unit of electricity in passing from A to B. 



Ans. Electric force from A towards B = 1 8 '85 dynes; work 

 done by unit in passing from A to B = 37 5 ergs. 



14. What is meant by the dimensions of a physical quantity ? 

 Deduce from the Law of Inverse Squares the dimensions of 

 electricity ; and show by this means that electricity is not a 

 quantity of the same physical dimensions AS either matter \ energy, 

 or force. 



15. Explain the construction and principles of action of the 

 quadrant electrometer. How could this instrument be made 

 self-recording ? 



1 6. One ot the two coatings of a condenser is put to earth, 

 to the other coating a charge of 5400 units is imparted. . It is 

 found that the difference of potential thereby produced between 

 the coatings is 15 (electrostatic) units. What was the capacity 

 of the condenser ? Ans. 360. 



17. What is the meaning of specific inductive capacity 1 Why 

 does hot glass appear to have a higher specific inductive capacity 

 than cold glass ? 



1 8. Compare the phenomenon of the residual charge in a 

 Leyden jar with the phenomenon of polarisation in an electro- 

 lytic cell. 



19. A condenser was made of two flat square metal plates, 

 the side of each of them being 35 centimetres. A sheet of 

 indiarubber "4 centim. thick was placed between them as a 

 dielectric. The specific inductive capacity of indiarubber 

 being taken as 2*25, calculate the capacity of the condenser. 



Ans. 1095*8 electrostatic units. 



