104: Lord Kelvin on 



come from any greater distance asunder D' to any smaller 

 distance D is 



V 2 A/1 1\ , , V 2 A 



-^r( D -5>), or approximately, -g^, 



if D is very small' in comparison with D'. 



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

 take V as | of a volt C.G.S. electromagnetic, or ¥ J^ C.G.S. 

 electrostatic. 



Let now A be 1 sq. centim. and D '001 of a eentim. 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 -12 grammes of water 1°. 



The table on the next page shows corresponding calculated 

 results for various distances ranging from 1/100 of a centim. 

 to l/10 lo of a centim. 



Columns 5 and 6 are introduced to illustrate the relation 

 between the electric attraction we are considering and che- 

 mical 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 tha disks ; as we see from the 

 table, at a distance of 1/100 of a centim. it amounts to only 

 1/400 of a milligramme-heaviness ; and to only 2J grammes- 

 heaviness at the distance 10~ 5 of a centimetre, which is about 

 ■J of the wave-length of ordinary yellow light. At the distances 

 10 -7 , 10~ 8 , 10 -9 of a centimetre the calculated forces of attrac- 

 tion are 25 kilogrammes, 2J tonsf, and 250 tons. This last 

 force is 2 or 3 times the breaking-weight per square centim. 

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



* Pellat's measured values range from '63 to "92, according- to the 

 physical condition left by less or more violent scrubbing with emery- 

 paper. The mean of these numbers is *77. Murray's range was still 

 wider, from -63 volt to 1*13, the smallest being for copper burnished, 

 opposed to zinc scratched and polished with glass-paper ; and the largest, 

 copper polished merely with emery-paper, opposed to zinc polished and 

 burnished. 



t The metrical ton is about 2 per cent, less than ("984 of) 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. 



