160 
MR, H. TOMLINSON ON THE INFLUENCE OF STRESS 
armatures employed; and, on the whole, when we consider that, as stated by them, 
the electromagnet had no apparent effect to make a piece of the wire set either 
axially or equatorially, w 7 e must regard their results with great suspicion.* 
I myself seven years ago tried the effect of an electromagnet on a copper wire coiled 
several times round a flat piece of hard wood, and placed between the poles of a 
powerful electromagnet actuated by 12 Grove’s cells, but in no case could I detect 
the slightest real alteration of resistance. The galvanometer then employed was only 
able to detect an alteration of 1 in 50,000 of the resistance, and I look forward with 
some interest to renewed experiments in the same direction with the much more 
sensitive instrument which I have at present. 
The Effect of Annular Magnetization on Electrical Resistance. 
Fig. 22. 
Let A B C D, fig. 22, be a thin slice of iron (in the plane of the paper), through 
which an electric current is passing in the direction of the arrows. The molecules at 
the upper part, A B, will tend to take up positions such that their axes are perpendi¬ 
cular to the plane of the paper and with their north ends above it. The magnetization 
imparted to the molecules will diminish from A B to the axis E F, where it will be 
zero, and below this axis the molecules will be impelled to place their south poles 
above the plane of the paper. If now H K be an independent current passing below 
A B C D, it will tend to reverse the magnetism of the molecules at C D and 
strengthen that of the molecules at A B; but since the lower molecules are nearer 
to H K than the upper ones, the total effect would be a partial diminution of the 
annular magnetism imparted by the current flowing through A B C D. If, on the 
contrary, the current below the wire flows from K to H, the annular magnetism would 
on the whole be increased. 
Now, some years ago, when making attempts to discover whether the resistance of 
an iron wire could be altered by passing a current above or below the wire, I was led 
to believe that such was the case. Among several other experiments, a knitting- 
needle provided with copper terminals was placed upon a strip of copper 16 inches 
long, 3 inches broad, and y^-th of an inch thick; the strip was well varnished, and 
the needle was laid upon it in such a position that the axis of the needle was 
* It seemed to be the more desirable to test these results, as Auerbach twice alludes to them in his 
paper (Phil. Mag., July, 1879, pp. 15-17), and should they not be correct, as I cannot help feeling is 
probably the case, others might be misled, owing to the well-deserved reputation of these experimenters. 
