540 Lord Kelvin [May 21, 



Hence, 



SttD'' 



Hence the work done by electric attraction in letting them come from 

 any greater distance asunder D' to any smaller distance D is : — 



V2 A / 1 I \ . , , V2 A 



if D is very small in comparison with D'. 



§ 27. For clean sand-papered copper and zinc * we may take V as 

 J of a volt c.g.s. electromagnetic, or :j^ c.g.s. electrostatic. 



Let now A be 1 sq, cm. and D, 'OOl of a centimetre. We find P 

 equal to • 249 dyne, and the work done by attraction to this distance 

 from any much greater distance is -000249. This is sufficient to 

 heat 5' 9 X 10"^^ grammes of water, 1°. 



The table on the next page shows corresponding calculated results 

 for various distances ranging from 1/100 of a centimetre to 1/10^^ of 

 a centimetre. 



Columns 5 and 6 are introduced to illustrate the relation between 

 the electric attraction we are considering and chemical affinity as 

 manifested by heat of combination. The " brass " referred to is an 

 alloy of equal parts of zinc and copper, assumed to be of specific 

 gravity 8 and specific heat * 093. 



§ 28. It would be exceedingly difficult, if indeed possible at all, 

 to show by direct experiment, at any distance whatever, the force of 

 attraction between the discs ; as we see from the table at a distance 

 of 1/100 of a centimetre it amounts to only 1/400 of a milligramme- 

 heaviness ; and to only 2 J grammes-heaviness at the distance 10"^ of 

 a centimetre, which is about ^ of the wave-length of ordinary yellow 

 light. At the distances 10"^, 10"^, 10'^ of a centimetre the calculated 

 forces of attraction are 25 kilogrammes, 2J tons,! and 250 tons. This 

 last force is 2 or 3 times the breaking weight per square centimetre 

 of the strongest steel (pianoforte wire), 6 times that of copper, 15 

 times that of zinc. We are, therefore, quite sure that the increase of 

 attraction according to the inverse square of the distance is not con- 

 tinued to such small distances as 10'^ of a centimetre; and at dis- 

 tances less than this, the electric attraction merges into molecular 

 force between the two metals. 



* Pellat's measured values rano^e from "63 to '92, according to the physical 

 coudition left by less or more violent scrubbing with emery paper. The mean of 

 these numbers is 17. Murray's range was still wider, from '63 volt to 1'13, the 

 smallest being for copper burnished, opposed to zinc vscratched and polished with 

 glass pa[ier; and the Jargest, copper polished merely with emery paper, opposed 

 to zinc polished and burnished. 



t The metrical ton is about 2 per cent, less than ('984 oO the British ton in 

 general use through the British empire for a good many years before 1890, but 

 destined, let us hope, to be rarely if ever used after the 19th century, when the 

 French metrical system becomes generally adopted through the whole world. 



