an Explanation of Hall's Phenomenon. 255 



the galvanometer. On pressing with the finger the middle 

 of one edge of the sandwich in a direction perpendicular to 

 its length and in the plane of the metal, though no appreciable 

 distortion was produced, the galvanometer was immediately 

 deflected, the direction of the current from the strip to the gal- 

 vanometer being always the same as the direction of the force. 

 A similar sandwich was prepared in which platinum-foil was 

 used instead of iron, and with this a transverse force produced 

 opposite ' deflections to those which occurred in the case of 

 iron. 



There is, of course, no doubt whatever, as to the real exist- 

 ence of a mechanical stress in a conductor through which a 

 current is passing, and which is at right angles to the lines of 

 magnetic force. Its effect may be rendered evident to the 

 eye by a simple experiment, to which I attach no importance 

 whatever except as affording a rather pretty illustration of a 

 perfectly well-known law. A shallow glass cell is fitted with 

 iron electrodes at its two ends, and is filled with mercury to a 

 depth of 3 or 4 millim. When this cell is placed between the 

 poles of an electromagnet and a current is passed through it, 

 the mercury along the transverse middle line is drawn towards 

 one side of the cell. Here it is heaped up, and, yielding to 

 the force of gravitation, it runs along the two ends of the cell 

 in the opposite direction, once more to be urged forwards 

 across the lines of force. The result of a continuation of this 

 process is the formation of two little whirlpools of mercury 

 which rotate in opposite directions in the two halves of the 

 cell. The same effect may be produced, but with less violence, 

 if a solution of sulphate of zinc is used with zinc electrodes. 



Having completely satisfied myself that mechanical strain 

 was concerned in the production of Hall's phenomenon, it 

 remained to ascertain the modus operandi. Thermoelectric 

 action again occurred to me, and I remembered that Sir 

 William Thomson had discovered that thermoelectric effects 

 occurred in stretched and unstretched wires of the same 

 material when brought into contact with each other. On 

 referring to Thomson's paper, which' was published in the 

 c Philosophical Transactions' for 1856, I found it stated that 

 if a stretched copper wire is connected with an unstretched 

 copper wire and the junction heated, a thermoelectric current 

 will flow from the stretched to the unstretched wire through 

 the hot junction ; while, if the wires are of iron, the direction 

 of the current is from unstretched to stretched. There could 

 be little doubt that effects of the same nature would occur in 

 sheets of these metals; and I proceeded to consider the precise 

 manner in which a sheet of metal conveying a current would 



