Papers on Contact Electricity. £7 



Our results as given above by Prof. Hoorweg are in terms 

 of a volt, and not in terms of the electromotive force of a 

 Daniell's cell, which is the standard employed by the pre- 

 vious observers. To reduce our numbers to the same stan- 

 dard, they must be diminished by about 10 per cent. They 

 become then : — 



Ayrton and Perry. 



Zn Pt . . 0-883) 



Cu Pt . . 0-214 V of a Daniell's cell. 



Fe Pt . . 0-332 J 



From these numbers it will be seen that while Kohl- 

 rausch, Hankel, and ourselves, although making the experi- 

 ments in quite different ways, and with necessarily dif- 

 ferent specimens of the metals, have obtained fairly consistent 

 results, those of Prof. Exner, with the exception of the 

 first, stand by themselves. Nevertheless all his results agree 

 with surprising accuracy with the numbers required by his 

 theory. 



IY. On page 597 of his paper, Prof. Exner refers to the 

 early experiments of Pfaff, which proved that the measured 

 electromotive force of contact of two metals was independent 

 of the gaseous medium surrounding them, provided, of course, 

 visible chemical action did not take place ; and Prof. Exner 

 states that the subsequent experiments of De la Hive, which 

 he thinks proved that chemical action was necessary, nega- 

 tived those of Pfaff. We have not been able to find any thing 

 about Pfaff in the reference given by Prof. Exner, which is 

 perhaps due to a misprint in the Annalen ; but Pfaff's letter 

 to Gay-Lussac, which we have come across in the Annales de 

 Chimie et de Physique, vol. xli. pp. 236-247, 1829, gives an 

 account of experiments made with as great accuracy as was 

 possible with the instruments in use at the time, and which 

 led to the result that the difference of potential, measured in 

 Yolta's way, between zinc and copper metallically connected 

 was not influenced by atmospheres of dry or damp air, oxygen, 

 nitrogen, carbonic-acid gas, hydrogen, and olefiant gas. This 

 is still one of the most valuable papers in the history of 

 contact electricity; and, with all due respect for Prof. Exner's 

 opinion, we are compelled to attach much more faith to the 

 careful work of Pfaff than to experiments of De la Eive, who, 

 we find, actually coated his plates thickly with varnish to pre- 

 vent oxidation, and, because with such plates he obtained but 

 little difference of potentials, concluded that the well-known 

 Volta effect was produced by oxidation. This experiment, 

 indeed, is only equalled in vagueness by the one described by 

 Prof. Exner at the end of his paper, made with silver in chlorine 



