306 ATMOSPHERIC ELECTRICITy. 



the air. From this it follows that, however decided may be the signs 

 of electricity which stationary apparatus exhibits at the approach of 

 certain clouds and at the beginning of rain, they usually disappear 

 when the sky is clear or more or less free from clouds. This apparatus 

 is then insufficient to study atmospheric electricity in clear weather ; 

 it can only show the existence of this electricity. Besides, in the case 

 where the radiation is active, they must frequently furnish uncertain 

 results, because the insulation of their supports, besides the difficulty 

 of rendering it perfect, varies with the hygrometrical state of the 

 air, the dust which settles on them, and a multitude of other secondary 

 causes. 



Beccaria, who devised a great many contrivances, avows that fre- 

 quently he was ignorant whether the absence of the electrical signs 

 proceeded from the fact that the air was deprived of electricity, or 

 because the humidity had destroyed the insulation. 



Another cause of error, to which M. Peltier* first called attention, 

 is found in the chemical action which the moisture exercise on the 

 wire or the oxydizable points of the apparatus. This scientist states 

 that among the apparatus which he set up he found one which con- 

 stantly gave an electrical current, because it was formed of a rod of 

 iron painted with oil. In moist weather the needle of a rhtometer 

 was deflected to 80° without there being any electrical action of the 

 atmosphere. It was to avoid this great inconvenience that M. Peltier 

 constructed of platina the parts of his apparatus which must remain 

 ■exposed to the action of the air. We may also remark that there are 

 multitudes of cases where the use of stationary apparatus mast be 

 difficult, as when we attempt to study atmospheric electricity on 

 mountains, in open sea, and generally in places where there are no 

 buildings. Perhaps, then, we might use the apparatus recommended 

 by M. Pfaff.t consisting of a long glass support, sustained by three 

 feet, and bearing a disk of wood, with which is connected, by means 

 of a hinge, a metallic stem composed of several pieces fitted to each 

 other. This apparatus may be placed in the open air in such a way 

 that its stem^ movable on its hinge, may take any desired inclination. 

 A simple metallic wire communicating with the stem serves to con- 

 duct the electricity to an electrometer. 



If the stationary apparatus is not well adapted to ascertain atmos- 

 pheric electricity in clear weather, it may still be very useful to 

 establish the electrical state of the clouds, especially in stormy 

 weather, provided care be taken to connect it at the same time with a 

 discharging conductor. In these circumstances, M. Peltier| made use 

 of a simple insulated copper wire, of about a hundredth ot an inch in 

 diameter, which was raised at one of its ends to about eighty feet above 

 the ground, and at the other plunged into a well forty feet deep. This 

 wire, wound with silk and covered with many coats of thick varnish, was 

 terminated above by a tuft of platina wire ; and the lower end, which 

 was plunged into the well, was also terminated by a wire of the same 

 metal. Towards the middle of the wire was interposed a rheometer 



* Ann. de Ch. et Phys., 3d series, torn. IV, page 414. 1842. 



t Dictionn. de Gehler, torn. VI, page 517. 



I Comptes Rendus, ttm. I, page 95, and torn. Ill, page 145, 1835 and 1836. 



