494 Mr. T. Gray on Measurements of the Intensity of the 



therefore allows both the change of moment M and the change 

 of effective length < 2a l to be determined. When the change of 

 moment only is required, this method has still an advantage 

 in the fact that the effect of want of exact compensation 

 between the two coils can be eliminated by means of the two 

 measurements. 



Sufficient sensibility is, for most purposes, obtained by 

 using a deflection within the limits of the scale, but almost 

 any degree of sensibility may be obtained by using an inferred 

 zero. It is not advisable to place the magnets very near to 

 m, because, if the tangent method be adopted, the compen- 

 sation of the coils will not then be perfect unless m is always 

 brought to its original zero position ; and if this objection be 

 got over by using a sine method, in which the whole appa- 

 ratus, including the magnetometer, can be turned round a 

 vertical axis, there still remains the objection that the effect 

 of distribution becomes very pronounced. An inferred zero 

 method therefore involves, when small magnets are being 

 tested, a diminution, by artificial means, of the intensity of the 

 field at m. This, however, cannot be pushed far, on account 

 of the very inconvenient fluctuations of zero which then take 

 place, due to changes of declination in the earth's field, com- 

 bined with the small variations of intensity of "H" being 

 superposed on a field, the whole strength of which is not large 

 compared with these variations. 



The results of these experiments, given in Table II., show 

 that it is of great importance, so far as induction is concerned, 

 that the length of the magnets should be at least forty times 

 their diameter, and that they should be made as hard as 

 possible. It appears that for the steel employed in the experi- 

 ments, which was of the kind commonly called silver steel, a 

 much stronger magnet is obtained with a blue than with a 

 glass-hard temper*. The magnets were originally magnetized 

 by placing them between the poles of a large Ruhmkorff 

 magnet excited by 24 tray-cells. They were again magne- 

 tized by placing them between the poles of the same magnet 



* Note added October 26, 1885. — Since this paper was in type I have 

 received a copy of No. 14 of the ' Bulletins ' of the United States Geolo- 

 gical Survey, which is wholly devoted to a discussion of experiments on 

 the " Physical Characteristics of the Iron-Carburets," by C. Barus and 

 V. Strouhal. In this work a large number of interesting experiments on 

 the effect of annealing specimens of this " silver steel," which had been 

 previously tempered glass-hard, are quoted. Among other things the 

 large increase of magnetic moment which is obtained by softening the 

 steel, up to a certain point, is clearly brought out. — T. G. 



