ATMOSPHERIC ELECTRICITY. 301 



grees of increase or diminution wliicli occur in its index. Scliiibler has 

 also remarked that the electrometer is charged much more quickly 

 when, instead of tinder, there is fixed on the point of the stem a wick 

 prepared with sulphur. 



The discovery of electro-magnetism having led to the construction 

 of the rheometer, this instrument was soon applied, in the hands 

 of philosophers, to ascertain the electrical phenomena of the atmos- 

 phere. Its first use as an electro-atmospheric apparatus was by M. 

 Calladon.* After having demonstrated, in 1820, that an electrical 

 machine might, like the pile, produce a current capable of deflecting the 

 magnetic needle, this scientest showed that the rheometer might be- 

 come a means of determining the quality of electricity which passes 

 into the conductors used in the researches on atmospheric electricity. 

 He raised on the observatory of the college of France a pole thirty- 

 six feet long, supporting a conducting wire covered with silk, and 

 terminated by two fine needles diverging a little from each other. 

 This wire passed through a glass tube and descended into a chamber 

 where two rheometers were placed with two needles, one of 100 and 

 the other of 500 coils, the wire of which was doubly covered with silk, 

 and each series of coils separated by taffeta, prepared with gum. He at- 

 tached the conducting wire to one end of the wire of one of these instru- 

 ments, while the other end communicated with the ground. By means 

 of this arrangement he obtained deviations of the magnetic needle, 

 not only during storms, but also in days when the sky was merely cloudy. 

 M. Peltier and M. Clarke, f of Dublin, also used this instrument with 

 advantage, especially for studying the distribution of the electricity 

 in thunder clouds. They used a rheometer of 8,000 coils, the wire of 

 which was covered with silk, and also with a coating of gum-lac var- 

 nish, which insured the best insulation. 



JResearches on atmospheric electricity have also been made with 

 other instruments than those we have mentioned. In 1704, ReadJ 

 used for this purpose the electrical condenser, an apparatus described 

 first by Bennet, and afterwards improved by Isicholson. It was 

 designed, by the aid of a movable disk, to render a small portion of 

 electricity perceptible by increasing the quantity T)f electricity con- 

 tained in two other similar and opposed stationary disks. From ex- 

 periments with this instrument Read concluded that the atmosphere 

 when entirely pure is almost always in a state of positive electricity, and 

 that when it is vitiated, whether by putrefaction of vegetable matters or 

 b}' that of animal substances, its electricity becomes negative; and finally 

 that respiration alone renders the electricity of the air negative. These 

 results, obtained by means of the doubler, have not been confirmed by 

 experiments made with other instruments ; on the contrary, it has 

 been discovered that they must be attributed not to atmospheric elec- 

 tricity, but to other causes derived from the imperfections of the in- 

 strument. The use of that apparatus, therefore, has been abandoned 

 in all researches on electricity of this character. 



In 1809 De Luc exhibited to the Royal Society of London, under the 



'^ Ann. (Ic Chim. et de Phys., torn. XXXIII, p. 62, 1826. 



t Comptes Rendus, torn. Ill, p. 145, 1836 ; and Philos. Magazine, torn. XVI, p. 224, 1840. 



j Joum. de Phys., torn. XLV, p. 468, 1794. 



