212 Mr. Prideaux's Experimental Contributions 



tin plate, to keep the sheet tin from touching it, whilst their 

 surfaces should be fully exposed for radiation, at an extremely 

 small distance. No deflection ensued on thus replacing the 

 sheet tin ; but on putting a slip of tin-foil in contact with both, 

 the needle started off' 25°. 



A plate of polished copper was similarly used, first with 

 sheet tin, afterwards with the tin plate ; but with no effect 

 whatever, in either case, until metallic contact was made. 



The total inefficacy of such large radiating surfaces, added 

 to the result of the preceding experiments, discourages the 

 hope of tracing the cause of thermo-electricity to any super- 

 ficial action analogous to radiation; and more convincing 

 evidence to the same effect will appear (20. et seq.). 



In fact I have been unable to find any table of radiation 

 corresponding with Becquerel's thermo-electric order. 



IV. Is the proximate cause of Thermo-electricity in any way 

 connected with Conduction ? 



12. Having previously found reason, from the experiments 

 of others as well as my own, to conclude that the heat pro- 

 duced by electricity in metallic conductors (and, with due 

 allowance, in liquids and air,) is in the order of, and probably 

 in proportion to the resistance it encounters in the body heated, 

 whether that resistance was owing to inferior conducting pro- 

 perty or diminished thickness, I was led by the following ex- 

 periment, compared with some of Becquerel's, to suspect a 

 reciprocity of action, such that electricity, restricted in its 

 passage, producing heat; so heat, allowed to flow more freely, 

 might produce electricity. 



The circuit from a voltaic coil was divided into 5 parts, 

 the two ends next the coil, and the 5th or middle piece, being 

 of stout copper wire, the ends each 2 feet, the middle piece 

 4 feet long ; the connexions from each end to the middle piece 

 were fine silver wire T Jo tn °f an mcn thick. Thus no electricity, 

 positive or negative, could reach the middle piece except 

 through these fine wires. 



The charge being now made such as to warm the silver 

 wires, a magnetic needle was moved along the circuit, which 

 was laid (ap ortion of the middle piece, of course, excepted,) in 

 the magnetic meridian. Its equidistance from the conducting 

 wire was ensured, by the card, on which it was supported, be- 

 ing kept always in contact therewith. When over the copper, 

 either of the middle or end pieces, the divergence was 36° ; 

 when over either of the silver wires, 26°. Thus heat seemed 

 to be produced, by restriction of the current, on entering the 



