CHARACTERISTICS OF CONDENSERS. 451 



CHAPTER V. 

 MEASUREMENT OF CAPACITY. DIELECTRICS. 



1042. CHARACTERISTICS OF CONDENSERS. The capacities to be 

 measured are almost always those of condensers (78), the type of 

 which is the Leyden jar. They consist of two armatures, separated 

 by a dielectric, and raised to different potentials. Submarine or 

 subterranean cables, which consist of a system of conducting wires 

 immersed in a layer of gutta-percha, satisfy this condition, and 

 frequently possess great capacities. The conducting wire or the 

 core of the cable forms the internal armature ; the outer coating is 

 formed either of a metal sheath which protects the cable, or by the 

 earth or water in which it is immersed. 



Of all forms, the Leyden jar is that best suited for keeping a 

 charge of electricity. Certain qualities of glass appear, at ordinary 

 temperatures, to be absolutely impermeable to electricity. Franklin 

 kept electricity for several months in a Leyden jar, the neck of which 

 had been sealed. Sir W. Thomson repeated this experiment with a 

 sealed flask, the internal armature of which was a layer of sulphuric 

 acid ; the loss was inappreciable even after several years. 



The use of glass, however, makes the apparatus too fragile, and 

 it is not possible to obtain plates so thin as to have great capacity in 

 a small space. Condensers in sheets are then much better. Alter- 

 nate layers of tinfoil and of thin sheets of mica, or sheets of 

 paraffined paper, are superposed on each other, all the tinfoil sheets 

 of even order projecting on one side, and the odd ones on the other; 

 they are connected separately, and thus furnish in a small volume 

 very large surfaces close together. 



All solid or liquid dielectrics present, in a greater or less degree, 

 the property (which is still imperfectly understood) of absorbing elec- 

 tricity. If we take a charged condenser, the dielectric of which is 

 not a gas, and after having discharged it by connecting the armatures 

 for a few moments, again insulate the armatures, we observe that it 

 then spontaneously acquires what is called a residual charge, which 



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