ATMOSPHERIC ELECTRICITY. 309 



witli a point, only from four to seven feet above the ground, at the 

 time when stationary conductors of more than a hundred feet high 

 gave no indication. What we have given above furnishes a sufficient 

 explanation of this fact ; for the movable apparatus being under the 

 influence of an electricity which acts on them by induction, the elec- 

 trical equilibrium must be disturbed at every change of elevation. 

 Besides this, the stationary apparatus, as we have seen, however care- 

 fully constructed, loses in part its isolation when the air is moist, during 

 fogs, heavy dews, cold and rainy nights ; while a movable apparatus, 

 such as an electrometer furnished with a point, which we usually keep 

 in the house, and which is not exposed to tlie air except at the moment 

 when we wish to use it, always has a sufficiently good insulation. 

 This kind of apparatus is the only one which has yet led to any definite 

 results in regular observations, made under a clear sky, at the sur- 

 face of the earth. 



There are two ways of using the electrometer, affording different 

 indications, and which we shoukl be careful not to confound. If v/e 

 desire only to know the nature of the electrical influence, whether 

 under a clear sky or a cloudy one, let the conducting stem be termi- 

 nated by one or several points, or by an ignited body; the leaves, 

 after having been brought to an equilibrium, will slowly or rapidly 

 diverge by the repelling electricity without the necessity of moving 

 the instrument. But the electricity thus collected being wholly de- 

 pendent on the conductibility of the air, and consequently varying with 

 the moisture in that medium, the rain and even with the force of the 

 wind, this mode of experimenting will simply give the measure of the 

 repulsive electricity which the instrument is in a state to preserve in 

 consequence of its insulation, and not that of the electric induction to 

 which it is subjected. To obtain this latter, M. Peltier* directs that 

 the conducting stem be terminated by a ball, v/hich, at the same 

 time that it prevents the radiation of the electricity accumulated at 

 the lower extremity, increases the effect of the induction. It is clear 

 that, after having brought the instrument to an equilibrium at a cer- 

 tain height, the indications which it will give by its elevation or its 

 depression will depend only on the induction of the atmospheric elec- 

 tricity, and may consequently be used as a measure of it. This 

 method is that which M. Peltier used in his regular observations, and 

 it appears to be the only one suitable to ascertain the variety of the 

 electrical tensions at short intervals of time. When this scientist 

 wished to make an observation he ascended on a terrace and placed 

 his electrometer on a tablet raised about five feet ; he brought it then 

 to an equilibrium by touching the stem and the lower part with a 

 metallic wire ; then he descended into a room below the terrace, and 

 there placed the instrument charged with electricity opposite to that 

 of the atmosphere on a tablet designed for it, where he afterwards 

 read it. This series of operations could be made with such rapidity 

 that it required only eight seconds. . 



M. Peltier remarked that for these experiments he needed only to 

 use a ball of about three inches in diameter, and not a longer stem 



Annales de Chimie et de Physique, 3<1 scries, torn. IV, p. 400, et suiv. 



