PROBLEMS AND EXERCISES. 417 



electrolytic cell containing a solution of silver. The current 

 was kept on for one hour ; the deflection observed at the begin- 

 ning was 36, but it fell steadily during the hour to 34. Sup- 

 posing the horizontal component of the earth's magnetic force 

 to be "23, calculate the amount of silver deposited in the cell 

 during the hour, the electro-chemical equivalent of silver being 

 0011340. Ans. -0526 gramme. 



17. A piece of zinc, at the lower end of which a piece of 

 copper wire is fixed, is suspended in a glass jar containing a 

 solution of acetate of lead. After a few hours a deposit of 

 lead in a curious tree -like form ("Arbor Saturni") grows 

 downwards from the copper wire. Explain this. 



1 8. Explain the conditions under which electricity excites 

 muscular contraction. How can the converse phenomenon of 

 currents of electricity produced by muscular contraction be 

 shown ? / 



QUESTIONS ON CHAPTER IV. 



i . Define the unit of electricity as derived in absolute terms ' 

 from the fundamental units of length, mass, and time. 



2. At what distance must a small sphere charged with 28 

 units of electricity be placed from a second sphere charged with 

 56 units in order to repel the latter with a force of 32 dynes? 



Ans. 7 centimetres. 



3. Suppose the distance from the earth to the moon to be (in 

 .round numbers) 383 x io 8 centimetres ; and that the radius of 



the earth is 63 x io 7 centimetres, and that of the moon 15 x 

 io 7 centimetres ; and that both moon and earth are charged 

 until the surface density on each of them is of the average value 

 of io units per square centimetre. Calculate the electrostatic 

 repulsion between the moon and the earth. 



4. A small sphere is electrified with 24 units of + electricity. 

 Calculate the force with which it repels a unit of + electricity at 

 distances of I, 2, 3, 4, 5, 6, 8, and io centimetres respectively. 

 Then plot out the "curve of force" to scale; measuring the 

 respective distances along a line from left to right as so many 

 centimetres from a fixed point as origin ; then setting out as 



2 E 



