408 



Prof. D. E. Hughes. 



[Mar. 6 y 



surface against another may alter the true value. The electro- 

 magnetic method is however infinitely superior when observations 

 are made on solid bars, or tubes of different degrees of thickness, to 

 observe the influence of depth or thickness, in producing a perfect 

 return to neutrality after cessation of the inducing effect of the 

 coils. 



The third method was a chemical one, somewhat similar to that 

 employed by Jamin, except that as the object was to study the curves 

 of neutrality, the bars were of annealed steel, highly magnetised ira 

 the coils, and afterwards reduced almost to a zero, by vibrating them, 

 or beating them gently with a wooden mallet. We had by this means 

 aided the molecules to follow their inclination, as they do in soft iron y 

 for when a soft steel rod is in a state of vibration, its molecules are 

 comparatively free ; but they rigidly retain the true curve of neu- 

 trality when not vibrated. We are thus enabled by dissolving the 

 exterior in various dilute acids, and by taking repeated observations,, 

 to draw graphically the waves of opposing polarities, which have 

 produced external neutrality. 



The curves obtained by the different methods are identical in form. 

 The simplest and most accurate method is the first, as we can choose 

 a hard variety of iron, such as ordinary hoop-iron, and by slight 

 vibrations, or blows with a mallet, allow the molecules sufficient 

 freedom to form their curve before separating, and as the material is- 

 sufficiently rigid not to be influenced by mere contact, or even 

 frictional drawings, we have on each strip a perfect record of its 

 state, and can thus analyse the internal state of a neutral compound 

 bar. 



If we take a compound bar of the hoop-iron, and draw the lower side 

 over the south pole of a magnet, it will be found nearly neutral, or if 

 not sufficiently so, we can reduce it by slight blows with a mallet r 

 suppose the united bar gives still a remaining magnetism of 18° on 

 the magnetic balance, on separating the components and observing the 

 same ends we find the lowest (or the bar which had touched the- 

 magnet) 150° north polarity, the next may be slightly north or zero ; 

 the rest will have varying degrees of south polarity, from 60° to 10° r 

 the total of which exactly balances the north polarity of 150, less 18° r 

 which we already observed as the remaining magnetism. 



If we do not wish to approach a perfect neutrality, we should not 

 vibrate the rods. In this case we may have 75° of remaining 

 magnetism, and find on separating the strips, that we have on lower 

 strip 150° north, and the total opposing south polarity of the interior 

 but 75° south, leaving the remaining 75° of north polarity first 

 observed on the compound bar unbalanced. 



The mutual reactions between the magnetic molecules in a solid 

 l)ar are precisely similar to those between two or more separate bars y 



