ELECTRIC CHARGE AND THE CONDENSER. 135 



7. A condenser is to be built up of sheets of tin foil 12 centi- 

 meters X 15 centimeters. The overlapping portions of the 

 sheets are 12 centimeters X 12 centimeters. The sheets are 

 separated by leaves of mica 0.05 centimeter thick. How many 

 mica leaves and how many tin foil sheets are required for a one- 

 microfarad condenser? Assume the inductivity of the mica to 

 be equal to 6. Ans. Mica, 655; tin foil, 656. 



8. A condenser consists of two flat plates each one meter square 

 and at a distance of 0.5 centimeter apart. A rotating device is 

 arranged to charge this condenser by connecting it to a 15 volt 

 battery, and discharge it through a galvanometer 50 times each 

 second, and the galvanometer deflection shows an average 

 current of 1.33 X io~ 6 amperes. Find the capacity of the con- 

 denser and calculate the value of the factor G in equation (i) 

 of Art. 79. Ans. 884 X io~ 16 . 



Note. A rotating device for making connections as specified in the problem is 

 called a secohmmeter. 



9. A condenser is made of two flat metal plates separated by 

 air. Its capacity is 0.003 microfarad. Another condenser has 

 plates twice as wide and twice as long. These plates are sepa- 

 rated by a plate of glass (inductivity 5) which is four times as 

 thick as the air space in the first condenser. What is the capacity 

 of the second condenser? Ans. 0.015 microfarad. 



10. Two metal plates, 100 centimeters X ioo centimeters, are 

 separated by 2 centimeters of air. This condenser is charged 

 by a battery having an electromotive force of 2,000 volts. What 

 is its energy in joules? Ans. 0.000884 joule. 



11. A flat glass plate, inductivity 5, size ioo centimeters X ioo 

 centimeters X 2 centimeters, is slid between the metal plates 

 specified in problem 10, the battery being left connected to the 

 metal plates. What is the energy of the condenser after the 

 glass is in place? Ans. 0.00442 joule. 



12. The 2,000- volt battery is disconnected from the metal 

 plates specified in problem n after the glass is in place, and the 



