150 ATMOSPHERIC ELECTRICITY. 



being removed at pleasure. A moveable communication should also be provi- 

 ded between the pointed rod and a metallic bar continued to the ground, so 

 that in cases of thunder-storms, or at any other time when the electricity of the 

 air is so strong as to be attended with danger, it may be allowed to escape to 

 the earth by putting the pointed rod in communication with this conductor. If 

 it be desired to observe the electric state of the air when it is strongly charged, 

 the bar connecting the pointed rod with the conductor may be brought so near 

 the latter as to allow the chief part of the electricity to pass through it to the 

 ground ; and, at the same time, the connexion of the electroscope with the 

 pointed rod being preserved, a sufficient quantity of electricity will affect it to 

 indicate the species of electricity with which the air is charged. 



For occasional observations a convenient and portable apparatus may be 

 formed with a common fishing-rod, which is divided into several pieces capa- 

 ble of being united at pleasure, so as to form a single rod of considerable 

 length. To the extreme piece of this let a rod of glass, terminated by a fine 

 metallic point, be attached ; a metallic wire attached to this point is carried to 

 the. electroscope, which will thus receive the electricity collected by the point 

 of the rod. This rod may be elevated in any situation in which it is desired 

 to examine the electric state of the air. 



Various forms of electroscopes are used to observe atmospheric electricity. 

 Saussure used two fine metallic wires, each having a small pith-ball suspended 

 at its lower extremity, and having its upper end attached to a rod of metal in- 

 serted in the top of a square tube of glass about two inches in the side. The 

 two balls were suspended in contact in the interior of this tube, and the extent 

 of their divergence was measured by a scale drawn on one of the sides of the 

 tube. To the upper extremity of the rod supporting the wires was screwed a 

 pointed conductor, composed of three parts fitting into each other, each meas- 

 uring from three to four inches in length. 



This conductor, being elevated in the air, collected the electricity. To pre- 

 serve the electroscope from the effect of the weather a brass cup was provi- 

 ded, which was screwed upon the rod supporting the wires at the foot of the 

 conductor. 



This apparatus is usually affected sensibly by the electricity of the air, when 

 raised in the atmosphere to the height of ten or twelve feet above the head of 

 the observer. In order to compare numerically the intensity of the electricity 

 which produces different degrees of divergence of the wires, Saussure adopted 

 the following ingenious method. Having constructed two electroscopes as 

 similar to each other in all respects as possible, and removed the conductors 

 from them, he electrified one of them so as to produce a certain divergence, six 

 lines, for example, of the balls. He then brought into contact the metal rods 

 of the two instruments, so as to share equally between them the electricity 

 with which the first was charged. The divergence was now reduced to four 

 lines. Hence electric charges in the ratio of 1 to 2, correspond to divergences 

 of the balls in the ratio of 2 to 3. 



The second electrometer being discharged, and again put in communication 

 with the first, the remaining charge of the latter was again shared equally be- 

 tween them, so that the first remained charged with only a fourth of its original 

 electricity. The separation of the balls was now found to be 2-8 lines. By 

 continuing this process, a table was constructed by which the ratio of the in- 

 tensities of the electricity could always be approximatively inferred from the 

 extent to which the balls were separated. It is evident that such a table will 

 ) not be the same for all electroscopes. Each observer must, therefore, con- 

 ( struct, from immediate observation, a table suitable to the individual electro- 

 \ scope which he uses. 



