298 ATMOSPnEKIC ELECTRICITY. 



designating hj this that there was no electricity in the air, according 

 to his electrometer and the manner in which he used it. 



However important might be the improvements made by De Saus- 

 sure in Cavallo's electrometer, as applied to researches in atmospheric 

 electricity, still that instrument could not render very feeble degrees 

 of electricity perceptible. Volta conceived the idea of putting it in 

 communication with the condenser, and by this method he obtained a 

 perceptible quantity of electricity in cases where all other instruments 

 indicated nothing,* He also made a change in De Saussure's elec- 

 trometer. f He left off the balls of elder pith, and substituted for the 

 metallic wires two small straws suspended to very movable little me- 

 tallic rings_, by means of which they were fitted to the stem of the 

 instrument in such a manner as to be near each other in a state of 

 rest. These little straws which, when dry, have the advantage of 

 being much lighter than De Saussure's metallic wires, and presenting 

 for equal weights much more surface, were then enclosed in square 

 glass flasks, having on one side of them a scale divided into 0.5 of a 

 line, and designed to measure their divergence. In this manner Volta 

 formed electrometers, which, though differing in respect to their 

 sensibility, had degrees which corresponded in a given ratio for each 

 instrument through the whole extent of the scale. In that which was 

 the most delicate the straws were two inches long by the sixth of a 

 line broad ; and in varying their length and diameter, or in length- 

 ening the metallic wires by means of which they were suspended, he 

 obtained a second electrometer, in which a divergence of 0.5 of a line, 

 regarded as a degree, was equal to five degrees of the first. To render 

 these instruments capable of being compared, he united their stems by 

 a conductor, then communicated to them the same quantity of elec- 

 tricity, and noticed the divergences they exhibited at the same 

 moment. Repeating this operation a certain number of times, with 

 different charges, he had all the elements necessary to form a table 

 which allowed him to reduce the degrees of the least sensible electro- 

 meter to those of the one which showed the greatest sensibility. 



On comparing his electrometer with that fitted with metallic wires 

 and balls, Volta found that in the latter the divergences were not pro- 

 portional to the charges, as De Saussure had observed, while in the 

 former, when it is perceptible or moderately so, the divergence is regular 

 up to 26°. ytill further he noticed that if the straws were equal in 

 length, a perceptible difterence in their size caused scarcely any differ- 

 ence in the variations. Thus, with straws of the same length, the sizes 

 of which were one-eighth and one- quarter of a line, he found but one 

 degree of difference in twenty. He also observed that when the straws 

 are less than an inch in length the electrometer exhibits no longer a 

 regular divergence, and finally, that the flask must not be too large, 

 lest the internal air should retain too long the electricity which it takes 

 away from the straws each moment. 



* To obtain exact results by means of the condenser, M. Becquerel observes, plates of 

 brass should not be used, for it is impossiV)le to ji^uard ourselves against the electro-chemical 

 effects resulting from the action of the liquids which adhere to the fingers on the metal. 

 Gilded plates of copper, or rather gilded plates of glass, should be substituted. 



f Diction, de Gehler, tom. Ill, p. 6G5, and Lehrbuch der Meteorologie von L. F. Koemtz, 

 vol. II, pp. 398 and 400 ; Halle, 1832. 



