ON THE TIME-LAG IX THE MA.GNETrSA.TION OF IRON. 



305 



begins to he turned, and ~^A~ has now 



a, certain positive and finite value. 



Thus near the portion lying between 



the first and the second stage of the 



curve of magnetisation, the intensity of 



magnetisation increases in a greater 



ratio than that of a simple proportion. 



Therefore, if we denote the short 



interval which elapses before we can perceive the deflection of the 



image after " make," by r, the compensation may be exact for — £*r, 



but not sufficient for -£-t, where t denotes a second, for instance. 



The result is that the compensation is soon found to be insufficient, 



and there follows the deflection of the magnetometer in the positive 



direction (/. e., in that direction in which the magnetometer strives to 



drive it), which must be distinguished from that due to pure growth 



of magnetisation in a constant field. 



(2°) We may suppose all these disturbing actions to be over 

 before the magnetometer can manifest its motion to our eyes on the 

 scale, and hence accept the growth observed as a pure time-effect. 



It seems to me that in stronger fields in my experiments, the 

 circumstances are just such as are described in (1°). But even in that 

 case, some at least of the initial magnetisation must be pure growth. 

 How much, however, we can not tell ; possibly the greater portion 

 is of that nature. After all, what is called the immediate effect can 

 have no definite limit. For, since it denotes the amount of the 

 magnetisation compensated at the instant when the deflection of the 

 magnetometer can just be perceived, it will depends largely on the 

 constants of the particular magnetometer ; for instance, on its moment 

 of inertia, the co-efficient of damping, the controlling force, etc., as well as 

 on the co-efficient of self-induction of the particular solenoid. Speaking 



