ELECTRIC CHARGE. THE CONDENSER. 189 



120. An alternating electromotive force which is represented 

 by the ordinates of the zigzag line in Fig. 115 acts on a circuit 

 which contains a condenser. The resistance of the circuit is neg- 

 ligible, and the capacity of the condenser is 20 microfarads. 



time 



I/IOQ second 



Fig. 115. 



Plot the curve of which the ordinates represent the successive 

 instantaneous values of the current. 



121. Two parallel metal plates at a fixed distance apart with 

 air between are charged as a condenser and discharged through 

 a ballistic galvanometer. The plates are then submerged in tur- 

 pentine and again charged and discharged through a ballistic gal- 

 vanometer. The charging electromotive force is the same in 

 each case and the throw of the ballistic galvanometer is observed 

 to be 7.6 divisions in the first instance and 16.7 in the second 

 instance. Find the inductivity of the turpentine. Ans. 2.2. 



122. The metal core and metal sheath of a submarine cable 

 are separated by an insulating layer of gutta-percha and they 

 constitute the two plates of a condenser. One mile of a subma- 

 rine cable has a capacity of 0.06 microfarad. What is the ca- 

 pacity of 100 miles of the cable? Ans. 6 microfarads. 



123. A condenser is to be built up of sheets df tin foil 12 cen- 

 timeters X i 5 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. 



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

 air. Its capacity is 0.003 microfarad. Another condenser has 



