AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
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longitudinal mechanical stress, which must cause rotation of the molecules to a certain 
extent, but without magnetic polarity, actually, unless carried to a very great excess, 
produces decrease of resistance, we are probably right in conjecturing that the change 
of resistance resulting from magnetization is in a great measure due to the fact that 
the current used in the “ bridge ” is encountered by a set of molecular currents 
circulating all more or less in the same direction, and in planes more or less at right 
angles to the direction of the former current as the induced magnetism is greater 
or less. 
Relation between the “ Rotational Coefficient ” of Metals and the 
Alteration of Resistance produced by Mechanical Stress. 
E. H. Hall has discovered that when a strip of metal along which a current is 
passing is placed between the poles of an electromagnet in such a position that the 
lines of magnetic force are perpendicular to the plane of the strip, an electromotive 
force is developed in a direction at right angles both to the plane of the strip and the 
lines of force, and that thus the current is deflected. This deflection varies in amount 
and also in direction with different substances, and Professor Hall has recently read 
before the British Association a paper* on this subject, in which he gives a table 
showing the extent and direction of the deflection produced in several metals. The 
extent of the deflection in any substance depends among other tilings upon a certain 
constant designated by Professor Hall as the “ rotational coefficient.” In this table 
the sign + or — is prefixed to the number representing the coefficient according as 
the current is deflected in the same direction in which the conductor itself tends to 
move, or the opposite. Below is given Hall’s table, and appended to it the numbers 
representing the increase of specific resistance per unit temporary increase of length 
when this latter is produced by mechanical stress. A + sign prefixed to the numbers 
denotes an increase, and a — sign a decrease of specific resistance. 
* ‘Nature,’ Nov. 10, 1881. (Abstract of a note on tlie above subject read by Professor E. H. Hali, at 
the meeting of the British Association at York.) 
