512 Prof. Maxwell and Mr. F. Jenkin on the Elementary 



The capacity of a spherical conductor in an open space is, in 

 electrostatic measure, equal to the radius of the sphere, — a fact 

 demonstrable from the fundamental equation (17). 



Experimentally to determine s, the capacity of the conductor 

 in electrostatic measure, charge it with a quantity, q, of electri- 

 city, and measure in any unit its potential (47) or tension (49), e. 

 Then bring it into electrical connexion with another conductor 

 whose capacity, s v is known. Measure the potential, e ]} of s and 

 Sj after the charge is divided between them ; then 



g = se=(s-\ r s 1 )e li 

 and hence 



s=-^-s 1 (25) 



e—e x l v ' 



In this measurement we do not require to know e and e } in 

 absolute measure, since the ratio of these two quantities only is 

 required. We must, however, know the value of s { ; and hence 

 we must begin either with a spherical conductor in a large open 

 space, whose capacity is measured by its radius, or with some 

 other form of absolute condenser alluded to in the following 

 paragraph. 



41. Absolute Condenser. Practical Measurement of Quantity. 

 — As soon as the electromotive force of a source of electricity is 

 known in electrostatic measure, the quantity which it will pro- 

 duce in the form of charge on simple forms is known by the laws 

 of electrical distribution experimentally proved by Coulomb. 

 Simple forms of this kind may be termed absolute condensers. A 

 sphere in an open space is such a condenser, and the quantity it 

 contains is se (equation 23). A more convenient form is a sphere 

 of radius x, suspended in the centre of a hollow sphere, radius y, 

 the latter being in communication with the earth. The capacity, 

 s, of the internal sphere is then, by calculation, 



s=-^. (26) 



y—x 



By a series of condensers of increasing capacity we may mea- 

 sure the capacity of any condenser, however large. The compa- 

 rison is made by the method described above (40). Thus, the 

 practical method of measuring quantity in electrostatic measure 

 is first to determine the capacity of the conductor containing the 

 charge, and then to multiply that capacity by the electromotive 

 force producing the charge (43). 



42. Practical Measurement of Currents: — The electrostatic 

 value of currents can be obtained from equation (21) when e 

 and r are known, or from equation (19) when v and C are known, 





