Electrical Resistance of Vertically -suspended Wires. 499 



point we have a definite thing of known form which can be 

 examined by a series of microscopes, and the point up to 

 which it can be clearly seen observed for each. 



I have thought it worth while to bring this subject forward 

 in this very incomplete form, because there are already 

 results of interest and there is so much prospect of more, that 

 it is likely that Members may be glad to investigate some of 

 the questions raised. 



LYIII. On the Electrical Resistance of Vertically -suspended 

 Wires. By Shelford Bid well, M.A., F.R.S.* 



FROM the experiments to be described in this paper, it 

 appears probable that the electrical resistance of verti- 

 cally-suspended copper and iron wires alters to a small extent 

 with the direction of the current traversing them. If the wire 

 is of copper, the resistance is slightly greater when the cur- 

 rent goes upwards than when it goes downwards ; while, on 

 the other hand, the resistance of an iron wire is apparently 

 greater for downward than for upward currents f. 



The arrangement employed for exhibiting this effect is 

 shown in the annexed diagram. A wire, A B, of the material 



* Communicated by the Physical Society : read March 12, 1887. 



t Venturing to imitate the fanciful analogy used by Sir William 

 Thomson, who, in discussing the thermoelectric effect now universally 

 associated with his name, speaks of the " specific heat " of electricity, we 

 may perhaps also speak of the " specific gravity " of electricity, and say 

 that (like its specific heat) it is positive in copper and negative in iron. 



