37 Note 6: on the molecular constitution of air 



It is true that the mathematicians define the zero of potential as the 

 potential at an infinite distance from the finite system which includes the 

 electric charges. This, however, is not a definition of which the experimentalist 

 can avail himself, so he takes the potential of the earth as a zero accessible to 

 all terrestrial electricians, and each electrician "makes his own earth." 



The earth-connexion used by Cavendish is described in Art. 258. But when 

 the whole apparatus of an electrical experiment is contained in a moderate 

 space, such as a room, it is convenient to make an artificial "earth" by con- 

 necting by metal wires the case of the electrometer with all those parts of the 

 apparatus which are intended to be at the same potential, and calling this 

 potential zero. 



It appears by observation, that in fine weather the electric potential at a 

 point in the air increases with the distance from the earth's surface up to the 

 greatest heights reached by observers, and in all parts of the earth. It is only 

 when there are considerable disturbances in the atmosphere that the potential 

 ever diminishes as the height increases. Hence the potential of the earth is 

 probably always less than that of the highest strata of the atmosphere. 



If the earth and its atmosphere together contain just as much electricity 

 as will saturate them, and if there is no free electricity in the regions beyond, 

 then the potential of the outer stratum of the atmosphere will be the same as 

 that at an infinite distance, that is, it will be the zero of the mathematical 

 theory, and the potential of the earth will be negative. 



NOTE 6, ART. 97. 

 On the Molecular Constitution of A ir. 



The theory of Sir Isaac Newton here referred to is given in the Principia, 

 Lib. it, Prop. xxin. 



Newton supposes a constant quantity of air enclosed in a cubical vessel 

 which is made to vary so as to become a cube of greater or smaller dimensions. 

 Then since by Boyle's law the product of the pressure of the air on unit of 

 surface into the volume of the cube is constant; and since the volume of the 

 cube is the product of the area of a face into the edge perpendicular to it, it 

 follows that the product of the total pressure on a face of the cube into the edge 

 of the cube is constant, or the total pressure on a face is inversely as the edge 

 of the cube. 



Now if an imaginary plane be drawn through the cube parallel to one of 

 its faces, the mutual pressure between the portions of air on opposite sides of 

 this plane is equal to the pressure on a face of the cube. But the number of 

 particles is the same, and their configuration is geometrically similar whether 

 the cube is large or small. Hence the distance between any two given molecules 

 must vary as the edge of the cube, and the force between the two molecules 

 must vary as the total force between the sets of molecules separated by the 



