304 ATMOSPHEZIG ELECTRICITY. 



"witliout obtaining the slightest divergence in the gold leaves. The 

 effect was the same when he previously charged the electroscope with 

 a quantity of electricity, similar or contrary to that of the globe ; the 

 divergence given to the leaves was not in the least changed. The 

 same conclusion may be drawn from an experiment made by Mr. Far- 

 raday,* in which he saw no perceptible divergence in an electrometer 

 placed in air, which had been powerfully electrified in a small 

 insulated chamber, the wall of which, composed of conducting sub- 

 stance, communicated with a powerful electrical machine, which was 

 worked in an adjoining room. 



DeSaussuref was the first to observe that atmospheric electroscopes 

 varied in their indications, according as we raise or lower the instru- 

 ments ; he likewise recognized in these variations all the signs of elec- 

 tricity of induction, and not those of the permanent electricity of con- 

 tact. M. Erman,! of Berlin, in prosecuting these researches with very 

 delicate electroscopes, concluded, from his experiments, that the varia- 

 tions in the nature and intensity of the electricity, manifested by these 

 instruments were entirely independent of the surrounding air, and 

 resulted from the fact that all bodies — even those which are absolutely 

 in electrical equilibrium with the earth and the air, and which cannot 

 consequently produce any divergence, have yet, under the influence of 

 the earth, atmospheres of free electricity of their own which are modi- 

 fied mutually by the changes produced in their removal or approach. 

 M. Peltier§ has recently resumed the enquiry, and, while he has been 

 led to a new interpretation of the electrical phenomena of the atmos- 

 phere, he has shown, by decisive experiments, that it is by induction 

 solely and not by the contact of the air that the instruments become 

 electrified. The considerations which he adduces in support of his 

 opinion, and which he communicated to the Academy of Sciences, 

 at the sitting of February 8^ 1841, are the following : 



1. If the stem of an electrometer is surmouned by a polished ball, 

 and the instrument is brought to an equilibrium by touching the 

 covering plate and the stem at the same time, we may leave it exposed 

 to the agitation of the air under a clear sky for hours, keeping it at 

 the same height, without its exhibiting the least sign of electricity. 



2. If the stem ends in a point the electrometer sometimes takes a 

 little electricity after half an hour or more ; this time is much 

 shortened if the stem ends in a bundle of metallic points or in an 

 ignited body, as in the experiments of Volta. 



3. In case the instrument gives no indication, if we raise it a few 

 feet, the leaves immediately diverge positively ; if, on the contrary, 

 it is lowered the same distance below the point of equilibrium, its 

 leaves diverge negatively ; and every time that it is brought to the 

 height of equilibrium the leaves fall to zero. 



These experiments clearly demonstrate that the electricity exhibited 

 by the electrometer is, in fact, developed by induction ; for, if it were 

 otherwise, the air would communicate to this instrument when it is 



* Experimental Eesearches in Electricity, eleventh series, p. 5, 1838. 

 t Voyage dans les Alpee, tome II. sec. 795, p. 212. 

 X Journ. de Phys., tome LIX, p. 98. 1804. 

 §Coniptes Kendus, tome XII, p. 307. 1841. 



