under the Operation of Feeble Magnetic Forces. 241 



purely harmonic, but requires higher components for its ex- 

 pression. If we put these terms out of account as relatively 

 small, we must still regard the phase of 3 as different from 

 that of 1q. The term in sin 6 will show itself as an apparent 

 increase in the resistance of the coil, due to hysteresis, and 

 independent of that which may be observed even with very 

 small forces as a consequence of induced currents in the 

 interior of the iron. The augmentation of resistance now 

 under consideration may be expected to be insensible when 

 the extreme range of magnetizing force does not exceed one 

 tenth of the earth's horizontal force. 



In the absolute determination (p. 235) of the susceptibility 

 to very small forces of the hard Swedish iron wire (3*30 

 millim. diameter), the length (about 100 diameters) was 

 scarcely sufficient for an accurate estimate. Similar experi- 

 ments on a thinner wire (1'57 millim. diameter) of the same 

 quality of iron gave &=6*85, corresponding to /a = 87. This 

 is in the hard-drawn condition. After annealing the same 

 piece of wire gave a higher result, but in this case the obser- 

 vation is complicated by the assumption of the magnetic state 

 occupying a sensible time. The susceptibility applicable to 

 the final condition is as high as 22 '0, more than three times 

 as great as before annealing. Bat a lower number would 

 better represent the facts, when the small magnetic force is 

 rapidly periodic ; and it may even be that under forces of 

 frequencies such as occur in telephonic experiments, most of 

 the difference due to annealing would disappear. Such a con- 

 clusion is suggested by the slight influence of annealing in 

 the experiment described in Part II.,* where is determined 

 the increment of resistance of an iron wire due to the concen- 

 tration of a variable current in the outer layers. But the 

 matter is one requiring further examination under better ex- 

 perimental conditions. 



The sensitiveness of the magnetometer-needle in the ex- 

 periments directed to prove the constancy of susceptibility to 

 small forces, suggests the inquiry whether iron should be 

 used when the object is purely galvanometric. An attempt 

 to produce a sensitive galvanometer by hanging a mirror and 

 needle between the pointed pole-pieces of a large electro- 

 magnet, arranged as in diamagnetic experiments, was not 

 very successful. A better result was obtained with an astatic 

 needle system, and an electromagnet on a much smaller scale. 

 This was of horseshoe form, the core being of hard Swedish 



* Phil. Mag. Dec. 1886, p. 488. 

 Phil. Mag. S. 5. Vol. 23. No. 142. March 1887. S 



