Mr* Prideaux on the Theory of Thermo-electricity. 205 



tails of an experiment upon a larger scale, which I hope soon 

 to have an opportunity of executing, and which will detect 

 and measure the velocity of electricity in its passage through 

 a metallic conductor, though the rapidity of its transmission 

 may exceed that of light: this I have proposed to effect, by in- 

 creasing, in certain proportions, 1st, the velocity of the re- 

 volving mirror; 2ndly, the length of the conducting wire; and 

 3rdly, the accuracy of observing the deviation of the sparks 

 from a vertical line. If I succeed in this point, it is obvious 

 that we shall possess a means of directly measuring the rela- 

 tive conducting powers of metals, and of ascertaining numer- 

 ous particulars respecting ordinary electricity which we at 

 present have no means of determining. 



Intending in the ensuing session to submit to the Royal 

 Society all the results I have obtained, in reference to a 

 new optical means of measuring rapid motions, minute inter- 

 vals of time, and feeble intensities of light, I have hitherto 

 refrained from publishing any incomplete statement of them ; 

 but I regret that this delay should have occasioned my ex- 

 periments to be so far misunderstood, that one of the earliest 

 which suggested itself to me, and which I have always consi- 

 dered to be of primary importance in the series, should be pro- 

 posed elsewhere, several months afterwards, as an experiment 

 yet to be tried, and be represented also as having entirely 

 escaped my attention. I remain, Dear Sir, yours, &c. 



Conduit-street, Hanover-square, C. WHEATSTONE. 



August 2, 1833. 



XL. Experimental Contributions towards the Theory of Ther- 

 mo-electricity. By Mr. John Prideaux, Member of the 

 Plymouth Institution. * 



r T 1 HE discovery of Professor Seebeck, that a bar of antimony, 

 * or of bismuth, heated in contact with a copper or brass 

 wire, would affect the magnetic needle, was soon extended by 

 chemists in this country and on the Continent to all the other 

 metals which are of ready access ; and a table of the thermo- 

 electric order of these metals was soon published by Professor 

 Cumming, and found to differ from the voltaic order, and also 

 from that of conduction, whether of heat or electricity. 



Other experimenters presently discovered thermo-electric 

 currents in single masses of metal, which have been traced 

 out with curious results f. 



* Communicated by the Author. 



f [Papers on the thermo-magnetisra of homogeneous bodies, by Mr. 

 Sturgeon, will be found in Phil. Mag. and Annals, N.S. vol. x. pp. 1, 116, 

 &c— Edit.1 



