Sir W. Harris on the Laws and Operation of Electrical Force. 71 



carefully describes the manipulation requisite in the use of this instru- 

 ment, together with its auxiliary appendages. He considers this 

 electrometer, as an instrument of electrical research, quite invaluable, 

 and peculiarly adapted to the measurement of electrical force. 



14. Having fully described this electrometer, and the nature of its 

 indications, certain auxiliary instruments of quantitative measure, to 

 be employed in connexion with it, are next adverted to. 



First, the construction and use of circular and globular transfer 

 measures given in the preceding Table, by which given measured 

 quantities of electricity may be transferred from an electrical jar 

 (charged through a unit-jar from the conductor of an electrical 

 machine) to any given surface in connexion with the electrometer. 

 The electrical jar he terms a quantity -jar, the construction and 

 employment of which is minutely explained, as also the construction 

 and employment of the particular kind of unit-jar he employs. 



15. Two experiments (1 and 2) are now given in illustration of 

 this method of investigation. 



Experiment 1 developesthe law of attractive force as regards quan- 

 tity ; which is found to vary with the square of the number of cir- 

 cular or globular inches of electricity, transferred to a given surface 

 in connexion with the fixed plate of the electrometer, the distance 

 between the attracting surfaces being constant. 



Experiment 2 demonstrates the law of force as regards distance 

 between the attracting surfaces, the quantity of electricity being con- 

 stant ; and by which it is seen that the force is in an inverse ratio 

 of the square of the distance between the attracting plates, the plates 

 being susceptible of perfect inductive action. From these two expe- 

 riments, taken in connexion with each other, we derive the follow- 

 ing formula, Fa^; calling F the force, Q the quantity, and D the 



distance. It is necessary, however, to observe that this formula 

 only applies to electrical attractive force between a charged and neu- 

 tral body in connexion with the earth, the two surfaces being suscep- 

 tible of free electrical induction, both direct and reflected. 



16. The author now refers to several experiments (3, 4, 5, and 6), 

 showing that no sensible error arises from the reflected inductive 

 action of the suspended neutral disk of the electrometer, or from 

 the increased surface attendant on the connexion of the surface under 

 experiment with the fixed plate of the electrometer ; as also that it 

 is of no consequence whether the suspended disk be placed imme- 

 diately over the fixed attracting plate of the electrometer, or over 

 any point of the attracting surface in connexion with it. 



17. Having duly considered these preliminary investigations, the 

 author now proceeds to examine experimentally the laws of surface 

 and boundary as regards plane rectangular surfaces, and to verify the 



formulae C= X /SJ3> and E= — ; in which C = charge, E=inten- 



sity, S = surface, and B = boundary. 



For this purpose a series of smoothly-polished plates of copper 

 were employed, varying from 10 inches square to 40 inches long by 



