294 ATMOSPHERIC ELECTRICITY. 



Various methods were used by the early philosophers to ascertain 

 the nature of the electricity with which their apparatus was charged. 

 Franklin* at first made use of two Leyden phials, one of which was 

 charged with the electricity of the rod, and the other with that of the 

 electrical machine. He placed them on a table at three or four inches 

 apart, and watched the motions of a little ball of cork suspended 

 between the two knobs. If, for example, the two bottles had each of 

 them been positively charged, the ball attracted at first and then re- 

 2)elled by the one was equally repelled by the other ; if, on the con- 

 trary, the charges were of a different kind, the little cork ball was 

 alternately attracted and repelled by both, and thus performed a series 

 of oscillations between the two knobs. To discover the nature of the 

 electricity with which the electrical chime of bells was charged, he 

 brought a glass tube positively electrified near the edge of the bell 

 which communicated with the rod ; in case this had negative electri- 

 city the ringing was stopped ; if it was positive it continued and even 

 became more lively. Sometimes he suspended a little ball of cork 

 near the edge of the same bell, and by presenting to it a glass tube 

 charged with electricity, he determined the kind of electricity which 

 the bell had communicated. Other scientists had recourse to the 

 various appearances which the electric light exhibited, according as 

 it was produced by positive or negative electricity. For this purpose Le 

 Eoy t placed opposite to each other, in a little box having a narrow open- 

 ing, two small metallic conductors, insulated and each having a blunt 

 point, one of which was connected with the atmospheric apparatus and 

 the other with the ground. In cases in which the atmospheric charged 

 the rod in a perceptible manner, the electricity was conducted by a 

 metallic wire into the inside of the box, and on looking in through a 

 small window two lights could be seen at the ends of the two points. 

 If the point connected with the rod presented a tuft or plume, and the 

 other a luminous star, the observer concluded that the air or cloud 

 was positively electrified ; if, on the contrary, the former presented 

 a luminous star and the latter a plume, he inferred that the elec- 

 tricity of the atmosphere was negative. This method was also. used 

 by BeccariaJ 



In his numerous experiments on this subject Cavallo§ made use of 

 an electrometer, with a" graduated dial to measure the intensity of the 

 electricity. He wished to determine the proportion between the inten- 

 sity and the corresponding divergence of the instrument. For this 

 purpose he gradually brought a tin plate, on which was placed a small 

 quantity of bran, near to a conductor connected with an electrometer, 

 and at the same time to the string of a kite in full flight. He found 

 that when the pendulum of the electrometer stopped at six degrees 

 the conductor began to attract the particles of bran, at the distance 

 of three-fifths of an inch ; and when the pendulum was at twenty de- 

 grees the attraction took place at an inch and a quarter ; and, finally 



- Experiments and Observations on Electricity, pp. 113, 115. 

 f Journ. de Phys., annce 1774, p. 1. 

 JLetteredeir Ellectricismo, p. 107. Bologna, 1758. 

 §Traite' complet d'Electiicite, p. 277. 



