4 i2 'PROBLEMS AND EXERCISES. 



and then put back in their former positions, with what force 

 will they act on each other ? 



Ans. They repel one another with a force of I dyne. 



10. Zinc filings are sifted through a sieve made of copper wire 

 upon an insulated zinc plate joined by a wire to an electroscope. . 

 What will be observed ? 



* 1 1. ^Explain the principle of an air-condenser ; and state why 

 it is that the two oppositely charged plates show less signs of 

 electrification when placed near together than when drawn apart 

 from one another. 



12. There are four Leyden jars A, B, C, and D, of which A, 

 B, and D, are of glass, C of guttapercha. A, B, and C, are of 

 the same size, D being just twice as tall and twice as wide 

 as the others. A, C, and D, are of the same thickness of 

 material, but B is made of glass only half as thick as A or D. 

 Compare their capacities. Ans. Take capacity of A as I ; 



that of B will be 2 ; 



that of C will be f ; 



and that of D will be 4. 



13. How would you prove that there is no electrification 

 within a closed conductor ? 



14. What prevents the charge of a body from escaping away 

 at its surface ? 



1 5. Explain the action of Hamilton's mill. 



1 6. Two brass balls mounted on glass stems are placed half 

 an inch apart. One of them is gradually charged by a machine 

 until a spark passes between the two balls. State exactly what 

 happened in the other brass ball and in the intervening air up 

 to the moment of the appearance of the spark. 



17. Define electric density. A charge of 248 units of elec- 

 tricity was imparted to a sphere of 4 centims. radius. What is 

 the density of the charge ? Ans. I -6 1 nearly. 



QUESTIONS ON CHAPTER II. 



i. A dozen steel sewing-needles are hung in a bunch by 

 threads through their eyes. How will they behave when hung 

 over the pole of a strong magnet ? 



