ATMOSPHERIC ELECTRICITY. 311 



apparatus. In this case the radiation from the wire raised into the at- 

 mosphere may become so considerable that the needle moves rapidly 

 and even reaches its maximum of deviation. 



We shall finish this first part of the general subject by a succinct 

 summary of what we have said of the use of electro-atmospheric appa- 

 ratus : 



1. The electro-atmospheric apparatus becomes electrified by induc- 

 tion and not by contact of the air. It acts in the same way as if it 

 were placed under a body positively electrified ; consequently, it does 

 not indicate the absolute quantities, but the differences of one and the 

 same state. 



2. Stationary apparatus, on account of their immobility, must be con- 

 sidered as conductors in equilibrium ; they are insufficient for the study 

 of atmospheric electricity under a clear sky and in a dry time ; they 

 do not become electrified, except by their radiation, and even then 

 they do not indicate the nature of the electricity, except when their 

 insulation is perfect, and they are not influenced by electric currents 

 proceeding from the chemical action of the steam upon the oxydizable 

 j^arts whicli enter into their composition. 



3. Stationary apparatus in which the two conditions mentioned just 

 above are fulfilled is very well suited to observe the sudden or gra- 

 dual changes which take place in atmospheric electricity, during 

 stormSj and in general under the influence of clouds sufficiently elec- 

 trified. An insulated copper wire, covered with silk thickly varnished, 

 terminated at the end in j)latina, and arranged in the manner indicated 

 by M. Peltier, appears to be best suited to this case. 



4. Prudence alone should prevent the use of kites in stormy weather. 

 When the sky is more or less clear, this movable apparatus may serve 

 to study the variations which occur in the induction of atmospheric 

 electricity, when the kite is raised from the surface of the earth towards 

 the upj)er regions of the atmosphere. 



5. We may obtain under a clear sky remarkable effects, such as 

 very powerful shocks, by means of the kite, which, according to Mr. 

 Henry, we must attribute not to the direct influence of atmospheric 

 electricity, but to the phenomena of induction of an electric current on 

 itself. 



6. When it is desired by a series of observations to measure the 

 variations of atmospheric electricity, and obtain comparative results, 

 the electrometer t'urni.shed with a stem terminated in a ball, appears 

 to be the only suitable instrument. We put it carefnlly in equilibrium, 

 at a definite height, and then lower or raise it a constant distance. 

 Its indications may then serve to measure the atmospheric electricity. 



7. If the stem of the electrometer is terminated by one or several 

 points, or by an ignited body the instrument furnishes indications 

 which are dependent on the conductibility, or in other words on the 

 moisture of the surrounding air, 



8. The rheometers are not deflected, except by a continued current of 

 electricity, and their sensibility is far inferior to that of the electro- 

 meters. They are very useful in stormy weather and at all times 

 when the influence of atmospheric electricity is very strong. 



