1889.] On Time-lag in the Magnetisation of Iron. 



273 



magnetism). The magnetic force due to the solenoid may be taken 

 as approximately equal to the whole magnetic force (although the 

 rod was barely 100 diameters long, this length should be sufficient to 

 approximate to endlessness when one is dealing with very low values 

 of magnetic susceptibility). On this assumption, one scale division 

 of the galvanometer is equivalent to 0'0021 of || ; the initial instan- 

 taneous susceptibility, that is, the gradient d^/d^, is 9*9, and the 

 initial instantaneous permeability (^§J/<ZJ) is 125. This value has 

 been confirmed by a number of independent observations made with 

 the same piece of annealed wire, and with another piece cut from the 

 same hank and also annealed. Taking the magnetism acquired after 

 1 minute, the initial susceptibility as regards that is about L5. 



Precisely similar results have been obtained by reversing feeble 

 magnetic forces. So long as the forces are very small, the compensa- 

 tion for "reverse" is the same as for "make " and for " break," and 

 the creeping of the magnetism in any given time after make, break, 

 or reverse is nearly proportional to the amount of the preceding 

 change of magnetising force- 

 In the following experiments the magnetising force was raised to 

 higher values, at which this proportionality no longer held good. As 

 before, the compensating coil was adjusted for each current to balance 

 the effect of " make," the iron being demagnetised by reversals 

 immediately before the "make." When a stronger current was 

 applied, the coil had to be pushed nearer the magnetometer : but up 

 to forces of 0'3 c.g.s. or so, it was practicable to secure an instan- 

 taneous balance by doing so. Observations of the drift were taken 

 at 5 and 10 second intervals during 1 minute.* These are given 



Table I. 



Time after " make." 



Current. 



27 



62 



110 



161 



261 



seconds. 





Magnetometer + comp. coil. 









47 



107 



224 



395 



798 



5 



65 



145 



304 



525 



974 





72 



159 



327 



560 



1071 



15 



74 



165 



339 



573 



1089 



20 



77 



169 



344 



581 



1098 



25 



79 



171 



347 



586 



1104 



30 



79 



173 



350 



590 



1109 



40 



80 



175 



354 



595 



1116 



50 



80 



177 



355 



598 



1120 



60 



80 



177 



357 



600 



1124 



* To mate tho drift large the top of the wire was this time only 4 cm. from the 

 magnetometer. 



x 2 ■ 



