226 ON A NEW MAGNETICAL INSTKUMENT. 



yet in tracing the errors to their source. It is manifest, however, 

 that an instrument may be a good differential instrument, while it 

 is incapable of yielding absolute results; and there are special 

 reasons why this should be the case with the apparatus now under 

 consideration. Accordingly its failure in the latter respect, even 

 though established, would furnish no ground for despairing of its 

 success in the former. 



It is obvious that the apparatus is wholly free from the errors 

 belonging to magnetical instruments moving on a fixed axle ; and 

 the only doubt of its performance must relate to the changes of 

 induced magnetism in the iron bar. Thus it might be questioned, 

 before trial, whether such a bar receives in all cases an amount of 

 free magnetism proportional to the inducing force ; whether, 

 again, the minutest changes in the latter are accompanied by 

 corresponding changes in the former; and whether, lastly, the 

 changes thus produced are instantaneous, or, at least, demand no 

 appreciable time for their development. 



In the first experiments which I made, for the purpose of de- 

 termining these questions, the induced magnetism of the iron bar 

 was altered by means of a permanent magnet, placed in the same 

 right line with the bar, and at a known distance from it. The 

 effect produced upon the position of the suspended magnet being 

 observed, the distance was altered by a known amount, and a new 

 observation taken ; and so on, at many different distances. Then, 

 the law of action of the inducing magnet being known, we may 

 calculate the changes of deflection of the suspended magnet, on the 

 supposition that the changes of the induced force of the bar are 

 proportional to those of the inducing action, and then compare 

 them with the changes of deflection observed. The calculated and 

 observed results of many series of observations, taken in this 

 manner, were found to accord as nearly as the accuracy of the 

 observations themselves allowed. 



In making this comparison, however, it is necessary to take 

 into account the effect of the direct action of the fixed magnet upon 

 the suspended one. The axis of the former magnet being not far 

 from the vertical passing through the centre of the latter, its action 

 upon it and upon the iron bar follow, nearly, the same law ; so that 

 its direct effects upon the position of the suspended magnet are, 

 very nearly, proportional to those which it produces through the 

 medium of the induced force of the bar. On this principle the 



