322 Royal Society ;- - 



Appendix. 



In order that the different expressions, " potential," " electromotive 

 force," "electrostatic force," "pressure of electricity from a me- 

 tallic surface balanced by air," used in the preceding statement, may 

 be perfectly understood, I add the following explanations and defini- 

 tions belonging to the ordinary elements of the mathematical theory 

 of electricity. 



1 . Measurement of quantities of electricity. — The unit quantity 

 of electricity is such a quantity, that, if collected in a point, it will 

 repel an equal quantity collected in a point at a unit distance with a 

 force equal to unity. 



[In British absolute measurements the unit distance is one foot ; 

 and the unit force is that force which, acting on a grain of matter 

 during a second of time, generates a velocity of one foot per second. 

 The weight of a grain at Glasgow is 32*2 of these British units of 

 force. The weight of a grain in any part of the earth's surface may 

 be estimated with about as much accuracy as it can be without a 

 special experiment to determine it for the particular locality, by the 

 following expression : — 



In latitude A. average weight of a grain 



= 32-088 X (1 + -005133 X sin 2 X) British absolute units.] 



2. Electric density. — This term was introduced by Coulomb to 

 designate the quantity of electricity per unit of area in any part of 

 the surface of a conductor. He showed how to measure it, though not 

 in absolute measure, by his proof plane. 



3. Resultant electric force at any point in an insulating fluid. — 

 The resultant force at any point in air or other insulating fluid in 

 the neighbourhood of an electrified body, is the force which a unit of 

 electricity concentrated at that point would experience if it exercised 

 no influence on the electric distributions in its neighbourhood. 



4. Relation between electric density on the surface of a conductor, 

 and electric fores at points in the air close to it. — According to a 

 proposition of Coulomb's, requiring, however, correction, and first 

 correctly given by Laplace, the resultant force at any point in the 

 air close to the surface of a conductor is perpendicular to the surface 

 and equal to Aitp, if p denotes the electric density of the surface in 

 the neighbourhood. 



5. Electric pressure from the surface of a conductor balanced by 

 air. — A thin metallic shell or liquid film, as for instance a soap-bubble, 

 if electrified, experiences a real mechanical force in a direction per- 

 pendicular to the surface outwards, equal in amount per unit of area 

 to 2vp 2 y p denoting, as before, the electric density at the part of the 

 surface considered. This force may be called either a repulsion (as 

 according to the views of the eighteenth century school) or an attrac- 

 tion effected by tension of air between the surface of the conductor 

 and the conducting boundary of the air in which it is insulated, as it 

 would probably be considered to be by Faraday ; but whatever may 

 be the explanation of the modus operandi by which it is produced, it 



