420 PROBLEMS AND EXERCISES. 



20. Calculate (in electrostatic units) the capacity of a mile of 

 telegraph cable the core being a copper wire of 'iS centim. 

 diameter, surrounded by a sheathing of guttapercha -91 centim. 

 thick. \k for guttapercha = 2-46 ; one mile = 160,933 

 centims.] Ans. 2,118,671 units. 



21. A Ley den jar is made to share its charge with two other 

 jars, each of which is equal to it in capacity. Compare the 

 energy of the charge in one jar with the energy of the original 

 charge. Ans. One ninth as great. 



22. A series of Leyden jars of equal capacity are charged 

 ' ' in cascade. " Compare the total energy of the charge of the 

 individual jars thus charged, with that of a single jar charged 

 from the same source. 



23. Classify the various modes of discharge, and state the 

 conditions under which they occur. 



24. Suppose a condenser, whose capacity is 10,000 charged 

 to potential 14, to be partially discharged so that the potential 

 fell to 5. Calculate the amount of heat produced by the 

 discharge, on the supposition that all the energy of the charge 

 is converted into heat. Ans. '0096 of a unit of heat. 



25. How do changes of pressure affect the passage of electric 

 sparks through air ? 



26. Why are telegraphic signals through a submerged cable 

 retarded in transmission, and how can this retardation be 

 obviated ? 



27. How is the difference of potential between the earth and 

 the air above it measured ? and what light do such measure- 

 ments throw on the periodic variations in the electrical state of 

 the atmosphere ? 



28. What explanation can be given of the phenomena of a 

 thunderstorm ? 



29. What are the essential features which a lightning-con- 

 ductor must possess before it can be pronounced satisfactory ? 

 And what are the reasons for insisting on these points ? 



30. How can the duration of an electric spark be measured ? 



