6 ART 4. — K. HONDA AND T. TEEADA. 



74 cm. long was used ; for want of a sufficiently long coil, two 

 coils of the same dimensions were used, placing the one above 

 another. The length of each coil was 39.90 cm., the internal 

 diameter 5.80 cm., and 4~« = 393.5. The air gap between the 

 two coils was only 1.7 cm., so that the heterogeneity of the field 

 in this space was inconsiderable. The wire to be tested then lay 

 nearly in a uniform field. 



The compensating copper wire was chosen of such dimen- 

 sions as to produce an elongation nearly equal to that of the 

 specimen. The final compensation was always made by properly 

 adjusting the distance between the two knife-edges supporting the 

 horizontal beams, from which the wires were suspended. A cer- 

 tain weight was added to the pan, or removed from it, and the 

 distances of the knife-edges were adjusted, until either the addi- 

 tion or removal produced no rotation of the mirror. Since the 

 elastic elongation was not exactly a linear function of the 

 tension applied, a compensated system for a given initial and 

 additional weight caused a rotation of the mirror for a different 

 initial weight and the same additional w^eight. Hence the com- 

 pensation was made anew for every initial weight, 1 kilogram 

 always being used for the applied weight. It was, however, a 

 matter of no great ease to obtain an exact compensation. Hence 

 the slight deflection in no field was always recorded, and taken 

 into account in the calculation of the deflection due to the change 

 of elasticity by magnetization. To prevent shocks in putting on 

 the additional weight, the face of the initial weight was covered 

 by a mat of cotton wool. In this way, the reading of the deflec- 

 tion was greatly facilitated. 



In the above arrangement, the rotation of the mirror caused 

 by a change of temperature is very small, as it is affected only 



