ATMOSPHERIC ELECTRICITY. 



151 



Volta used, for a like purpose, an apparatus similar to that of Saussure, but 

 adopted the straw electroscope. He assumed that the angles of divergence of 

 the blades of straw within the limits of 26 are sensibly proportional to the in- 

 tensities of the electric charges, and that, provided the blades exceed an inch 

 or two in length, the results are^Vot affected by any small variation of length. 

 It is safer, however, to construBJP table according to the method explained 

 above, whatever be the form of the electroscope. 



To augment the sensibility of the instrument, Volta also fixed a lamp to the 

 point of the conductor, and interposed a condenser between the conductor and 

 the electroscope. Both of these expedients, however, render the indications 

 of the instrument uncertain. In the process of combustion electricity will be 

 liberated, the effects of which will combine with those of the atmosphere in af- 

 fecting the electroscope ; and unless the plates of the condenser be formed of 

 gold or platinum, or be coated with these metals, their oxydation, by the depo- 

 sition of moisture upon them, would produce disturbing effects. 



In some cases the multiplier, or galvanometer, is advantageously applicable 

 for meteorological purposes. Since, however, the electric current transmitted 

 through it in such applications has greater intensity than that which is pro- 

 duced in Voltaic arrangements, greater precautions must be taken to insulate 

 the wire. For this purpose the wire, wrapped in the usual manner with silk, 

 may be immersed in a concentrated solution of gum lac in alcohol. When well 

 coated with this varnish, the electricity will not escape from one convolution 

 to another. 



In the application of the multiplier to detect the electricity of the air, one 

 extremity of the wire is attached to the foot of a pointed insulated conductor, 

 elevated to the proper height in the atmosphere, and the other extremity com- 

 municates with the ground. The air and the earth being in opposite electrical 

 states, a current will pass through the wire, the intensity of which will be 

 indicated in the usual manner, by the deviation of the magnetic needle. 



OF THE ORDINARY STATE OF THE ATMOSPHERE. 



One of the earliest results of the observation of the electrical state of the 

 air was the discovery of the fact that in clear weather, when the natural state c 

 of the atmosphere is undisturbed by clouds, it is always charged with positive 

 electricity, and the surface of the earth is, on the contrary, charged with negative 

 electricity. Volta explained this fact by stating that in the evaporation of wa- 

 ter the natural electricity of the liquid is decomposed, the positive fluid esca- 

 ping with the vapor, and the negative fluid remaining on the vessel in which 

 the liquor is evaporated ; and this process going on upon a large scale in the ( 

 oceans, seas, and other large collections of water, might charge the atmo- ' 

 sphere with free positive electricity. But we have seen from Peltier's experi- 

 ment, that mere evaporation without chemical decomposition is not enough ; we 

 have seen, too, from Armstrong's and Faraday's experiments, that mere evapo- 

 ration without friction is not enough ; we are hence led to modify our views, 

 and consider how far chemical effects and friction can be included as operating 

 causes in the electrization of the atmosphere. 



It is certain that such essential chemical effects as the liberation of particles 

 of water of crystallization from combination with salts, do not exist in the 

 evaporation to which common consent has ascribed the electricity of the at- ' 

 mosphere ; and philosophers have felt that the cause here assigned is inade- \ 

 quate to the effect. If they tacitly accept the theory, it is rather for want of | 



better than from any feeling of conviction. They cannot imagine the con- * 



