37* Mr. W. Petrie's Results of Experiments 



Among the formulae of § 7. we may distinguish the equa- 

 tions (44.) (9.) and (51.), from which follow (as the author 

 observes, p. 95) the laws of rectilinear, circular and elliptic 

 polarization ; and it may be remarked that the equation (9.), 

 relative to a transparent medium, viz. 



f = A. cos (k z—st + \) rj = B cos (kz—st+p) 

 £ = Ccos (kz— st + v) 



is deduced from the general equations which represent the 

 infinitely small motions of a system of molecules that is from 

 the equations (16.) of the memoir on Dispersion, for obtaining 

 which the author has made no particular assumption, and 

 has no occasion to make any term vanish. He also observes 

 (p. 83) that we cannot in general reduce the equations (9.) 

 to those which represent rectilinear polarization, and in which 

 A, /a, v are equal. 



The equations (16.) of the memoir on the Dispersion are 

 the differential equations of motion in their most general 

 form, corresponding to those of art. 59. in my tract. 



The reader will immediately perceive the relation which 

 these researches bear to the general subject. These last re- 

 sults accord exactly with mine obtained by a different method, 

 as well as with those of the late Mr. Tovey, by whose recent 

 death the science of this country has sustained so great a 

 loss. 



Oxford, Oct. 3, 1841 . 



LIV. Results of some Experiments in Electricity and Mag- 

 netism. By William Petri e, Esq.* 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



T3EING desirous of obtaining certain information relating to 

 -""* electricity and magnetism, which I failed to find in various 

 works on these subjects at the time published, I was induced 

 to make experiments, from which the following are the con- 

 cise statements, showing the results, and which may, perhaps, 

 possess sufficient interest to find a place in your Journal. 



It is commonly stated, as by Lenz, that the conducting 

 power of wires varies as the section, and inversely as the 

 length; but this only refers to a particular case, namely, 

 when the amount of electricity which the battery can circulate 

 when the current is quite unimpeded (such as when the cir- 

 cuit is made with a very short thick wire), is very great com- 



* Communicated by the Author. 



