4 1 6 Note 25: calculated capacities of cylinder and -wires 



NOTE 25, ARTS. 360, 539, 666. 

 Capacities of the large tin Cylinder and Wires. 



The dimensions of the cylinder are given more accurately in Art. 539. It 

 was 14 feet 8-7 inches long, and 17-1 inches circumference. Its capacity when 

 not near any conductor would be, by the formula in Note 12, 22-85 inches, 

 and when its axis was 47 inches from the floor it would be 31-3 inches, or in 

 Cavendish's language 62-6 inches of electricity. Cavendish makes its computed 

 charge 48-4, and its real charge 73-6. See Art. 666. Now the charge of either 

 of the plates D and E was, by Art. 671, 26-3 inches of electricity, so that 



tin cylinder = 1-19 (D + E). 



The capacities of the different wires mentioned in Arts. 360 and 539 are, 

 by calculation, 



length diameter capacity 



29 J 2-67 



22 J 2-09 



37 "IS 3-13 



27-6 -15 2-46 



20-8 -15 1-88 



31 -15 2-71 



24 -15 2-28 



The ratio of the charge of the first of these wires to that of the second is 1-37. 



NOTE 26, ART. 369. 

 Action of Heat on Dielectrics. 



The effect of heat in rendering glass a conductor of electricity is described 

 in a letter from Kinnersley to Franklin* dated i2th March, 1761. He found 

 that when he put boiling water into a Florence flask he could not charge the 

 flask, and that the charge of a three pint bottle went freely through without 

 injuring the flask in the least. 



Franklin in his reply describes some experiments of Canton's on thin 

 glass bulbs, charged and hermetically sealed and kept under water, showing 

 " that when the glass is cold, though extremely thin, the electric fluid is well 

 retained by it." 



He then describes an experiment by Lord Charles Cavendish, showing that 

 a thick tube of glass required to be heated to 400 F. to render it permeable to 

 the common current. 



A portion of a glass tube near the middle of its length was made solid, and 

 wires were thrust into the tube from each end reaching to the solid part. The 

 middle portion of the tube was bent, so that a portion, including the solid part , 

 * Franklin's Works, edited by Sparks (1856), vol. v. p. 367. 



