Equijpotential Lines of an Electric Current. 285 



sends a given current through it is relatively large. Thus the 

 P. P. of bismuth is perhaps 13000 times as great as that of 

 gold and 800 times as great as that of nickel, but the equipo- 

 tential lines in bismuth may not be rotated more than 200 

 times as far as those of gold or more than 80 times as far as 

 those of nickel, in the same magnetic field. 



Summary. 



Experiments made with strips of steel, having longitudinal 

 slits, are in agreement with predictions made in this Journal 

 for February, 1885. 



The transverse current from a very short strip is smaller 

 than that from a long strip. (Already announced by Ettings- 

 hausen and Nernst.) 



The table of coincidences given by Mr. Shelford Bidwell in 

 his theory of the " Hall effect" is confirmed in the case of cop- 

 per, iron and zinc, but exceptions to it are found in " French 

 cold rolled steel" and aluminium. 



In silver and bismuth the P. P. appears to be independent 

 of the shape of cross-section of strips examined. (This con- 

 clusion as to bismuth already announced by Ettingshausen and 

 Nernst.) 



In the magnetic metals, iron, cobalt and nickel, the P. P. is 

 greatly dependent upon the shape of cross-section of the speci- 

 men examined, being, in differently shaped pieces of a given 

 metal in a comparatively weak magnetic field, apparently pro- 

 portional to the intensity of magnetic induction through the 

 respective pieces, and therefore much greater when the thick- 

 ness of the piece is large compared with the width than when 

 the opposite is the case. Therefore, although the mere pos- 

 session of great magnetic permeability does not directly insure 

 to a metal an especially large P. P., it may indirectly do so in 

 a properly shaped specimen by insuring intense magnetization 

 from a weak magnetizing force. 



In the case of cobalt and of nickel there is some evidence 

 that the P. P. attains a maximum value at a low or medium 

 stage of magnetization. (Ettingshausen and ISTernst make a 

 similar remark concerning cobalt, iron and antimony.) 



When a piece of iron, cobalt, or nickel is made to approach 

 the state of " magnetic saturation," the transverse current ob- 

 tained from it increases somewhat less rapidly than the mag- 

 netic induction through the metal, but experiments with very 

 highly magnetized iron and nickel indicate that this transverse 

 current tends toward a fixed limit rather than toward a maxi- 

 mum followed by a decline. 



The P. P. of bismuth appears to diminish continually as the 

 intensity of the magnetizing field rises from low to very high 



