310 ATMOSPHERIC ELECTEICITY. 



of the electrometer than twenty inches; he preferred even one of from 

 twelve to sixteen inches. A longer rod diminished the sensibility of 

 the instrument, because the electricity driven to the upper extremity 

 leaves to the opposite electricity the remainder of the length over which 

 it is to be distributed. Consequently the longer the stem the less is 

 the electricity which reaches the gold leaves. 



Instead of raising the instrument, he prefers to lower it below the 

 point of equilibrium, because it is not convenient, and frequently it is 

 even impossible to read exactly the number of degrees which the 

 apparatus indicates while the air is charged with resinous vapors, 

 when it is necessary to raise it one or two metres above the same point. 

 M. Peltier remarked, however, that the divergence of the leaves or the 

 deviation of the needle of his electrometer is less on lowering the 

 instrument than on raising it; so that we ought to fix the proportion 

 in order to establish the direct tension in the formation of the tables 

 which it may be desirable to prepare. The reading was made in a 

 room, and not in the open air, to avoid the inconveniences of the 

 ■wind, rain, the heat of the sun, and especially the error which may 

 result from the electric tension which the head of the observer receives 

 in the open air, and which might act on the instrument and compli- 

 cate its indications. Finally, to avoid alike the induction of the hand 

 on the stem, this is touched in the lowest part with a fine metallic 

 wire. 



Besides electrometers, science also has rheometers to measure the 

 intensity of atmospheric electricity; but, as M. Peltier* remarks, 

 two things limit the use that can be made of them. First, a great 

 quantity of electricity in motion is necessary to produce deviation in a 

 rheometer, while a small portion is sufficient to determine the di- 

 vergence of the gold leaves of an electrometer. M. Peltierf proved 

 that the escape of a quantity of electricity, representtd by a devia- 

 tion of 7.069 degrees of his static electrometer, was needed to produce 

 a single degree of deviation in an excellent rheometer of 3,000 coils 

 of a wire of 0.15 millimetres, having a system of light needles of 

 five centimetres in length. Then, also, it was necessary for the devi- 

 ation of this instrument to apply a continued current, and not a succes- 

 sion of small discharges. It therefore follows that the rheometer gives 

 no indication during the dryest and most electric days, if the extremity 

 of the wire is not raised to a great height in the air, while in the 

 last case the electrometer exhibits a very powerful electricity. Thus 

 it is that M. Peltier observed in his experiments under a clear sky, 

 that the deviation of his rheometer did not begin to be produced till 

 the kite with which one of the ends of the wire of the instrument 

 was connected had reached the height of one hundred feet above the 

 ground, while his electrometer indicated an increasing positive elec- 

 tricity beginning from the height of ten iieet. 



If the rheometer gives no indication under a clear sky, when the 

 extremity of the wire is placed. at a small height above the ground, 

 it is not so when clouds and especially thunder gusts pass over the 



•■•■ Comptes Eendus, torn. X, p. 712. 1840. 

 I Annales de Chimie et de Physique, torn. LXYII, p. 440. 183S. 



