558 JAMES CLERK MAXWELL. 



insulated, when it was found that there was sufficient positive 

 electricity on the outer sphere to deflect the electrometer 

 through more than 300 times the largest deflection which 

 could escape notice. Now when the brass ball was insulated, 

 the negative charge upon it was about -^th of the charge 

 originally on the copper sphere, and this induced a positive 

 charge on the sphere when in connection with the earth, 

 which was about -^^ of its original charge. Hence ^-J-^ of 

 the original charge is more than 300 times the largest charge 

 which could escape observation. From these figures it fol- 

 lows that, in the expression of the law of electrical action, 

 the true index of the inverse power of the distance must be 

 either 2 or differ from it by less than 2]L 600- These experi- 

 ments were carried out in the Cavendish Laboratory by Mr. 

 MacAlister of St. John's College. In note 19 appended to 

 the Cavendish papers, Maxwell describes the experiment, 

 and gives its complete mathematical theory. 



The idea that electricity flowing in conductors behaves 

 like an incompressible fluid is at least as old as Cavendish ; 

 but before he had the opportunity of reading Cavendish's 

 papers, Maxwell taught that all electric discharges involve a 

 displacement of electricity in a closed circuit, the electricity 

 behaving both in conductors and in dielectrics like a per- 

 fectly incompressible fluid. Thus, when a Leyden jar is 

 discharged, a certain quantity of electricity flows from the 

 inner coating through the knob of the jar, and hence to the 

 outer coating, but an equal quantity also passes across any 

 surface which we may imagine drawn within the glass, so as 

 to include the inner coating and exclude the outer. The 

 glass, therefore, may be regarded as completing a circuit across 

 every section of which the same quantity of electricity flows. 

 Again, if positive electricity be communicated to a conductor 

 by means of a wire which serves as an electrode, an equal 

 quantity passes out from the surface of the conductor and is 

 squeezed into the surrounding dielectric, causing a similar 

 transfer of electricity across every surface drawn in the 

 dielectric so as to surround the conductor, the extent of the 

 displacement of course diminishing as we recede from the 



