POTENTIAL AND CAPACITY 95 



of the wire P is brought to the same level as its surroundings, 

 or at least to the mean level so that each layer takes with it equal 

 and opposite quantities. 



If the electrometer is at lower potential to begin with than the 

 neighbourhood of P, it is evident that negative will go off, while 

 the electrometer will be positively electrified. 



This process has been realised practically by various methods. 

 In the method first used a burning match was fixed at one end of an 

 insulated conducting rod, which was brought to the given point, the 

 other end being connected to the electrometer. The hot mass of the 

 flame is a conductor, and as it is continually being thrown off it 

 carries electrification with it as long as there is any difference of 

 potential between the end of the rod and the surrounding air. 



In a second method, still sometimes used, an insulated can of 

 water discharges through a fine nozzle drop by drop. In Lord 

 Kelvin's original description of this instrument as applied to find the 

 potential outside a window he says : " With only about ten inches 

 head of water and a discharge so slow as to give no trouble in re- 

 plenishing the can of water, the atmospheric effect is collected so 

 quickly that any difference of potentials between the insulated 

 conductor and the air at the place where the stream from the nozzle 

 breaks into drops is done away with at the rate of 5 per cent, per 

 half second, or even faster. Hence a very moderate degree of 

 insulation is sensibly as good as perfect so far as observing the 

 atmospheric effects is concerned." (Electrostatics and Magnetism, 

 p. 200). 



It is usual now to employ a wire tipped with radium. The air 

 is ionised by the radium, and the charge on the end of the wire is 

 neutralised by the ions of opposite sign. 



Atmospheric electricity. Either of these instruments may 

 be employed to determine the potential at any point in the air 

 with regard to the earth. It is found in general that the potential 

 rises upwards from the surface, especially in clear weather. This 

 of course implies negative electrification of the earth's surface, the 

 corresponding positive electrification being scattered through the 

 air above the surface. When clouds are formed they act as 

 conducting masses and become electrified on their surfaces. If the 

 weather is not stormy the potential in the air is still usually 

 positive with regard to the earth, the under surface of a cloud 

 being probably positively charged and its upper surface negatively 

 charged. But in stormy weather it frequently happens that the 

 earth is positively charged under a cloud. This may possibly be 

 explained on the supposition that the cloud was electrified by 

 induction bv the ground under it and that the positive charge on 

 the cloud nas been removed by the rain falling from it; the 

 negative then spreading over the whole might produce a negative 

 potential in the air under it. We cannot here go into details of 

 the very puzzling subject of Atmospheric Electricity, a subject in 



