-1859] WRITINGS OF JOSEPH HENRY. 321 



The popular idea in regard to this is that the atmosphere 

 at such times contains more electricity than at others ; but 

 these appearances are not due to the variation of the elec- 

 tricity in the atmosphere, but simply to the less amount of 

 vapor which is present. When the clothes are rubbed to- 

 gether one part becomes positive and the other negative, and 

 in dry air the excitement increases to such an intensity that 

 the restoration of the equilibrium takes place by a visible 

 spark; but when the air is moist, the equilibrium is silently 

 restored as soon as it is disturbed, and no excitation is per- 

 ceptible. 



Similar effects are observed on the dry plains of the western 

 part of our continent : in rubbing the horses or mules, sparks 

 of electricity may be drawn from every part of the body of 

 the animal. Persons in delicate health, whose perspiration 

 is feebly exhaled, sometimes exhibit electrical excitement in 

 a degree sufficient to surprise those who are not familiar 

 with the phenomena. But these exhibitions have no con- 

 nection with animal electricity, and are merely simple illus- 

 trations of the electricit}'' developed b}' friction in an atmos- 

 phere too dry to permit the usual immediate and silent res- 

 toration of the electrical equilibrium. 



Distribution of Electricity. — The mutual repulsion of the 

 atoms of electricity, varying inversely as the square of the 

 ■distance, gives rise to the distribution of the fluid in regular 

 geometrical arrangements, the form of which may be calcu- 

 lated with mathematical precision. As one of the simplest 

 cases of distribution, suppose a conductor of the form of a 

 cylinder, with hemispherical ends (for example, one of wood, 

 covered with tin foil) to be suspended horizontally in dry air 

 with silk threads, and thus insulated to be slightly electri- 

 fied by touching the middle of it with a charged body ; the 

 atoms of the fluid, by their mutual repulsion, will separate 

 as far as possible from each other, and be found at the two 

 extremities. If the conductor were not surrounded with a 

 non-conducting fluid, like the air, they would be driven off 

 by the same repulsion into space, and thus indefinitely 

 separated. 



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