288 PRELIMINARY. 



opposite, the resultant moment is null, and the equilibrium is neutral 

 in any position whatever in a uniform field ; such a system is said to 

 be astatic. 



If the moments OA and OA' are almost equal and make an 

 angle nearly equal to 180, the resultant OB is very small and is 

 directed sensibly in the direction of the line bisecting the angle 

 AOA'; it is therefore perpendicular to each of the needles. Thus, 

 when two magnetised needles, forming a quasi astatic system, are 

 in a uniform magnetic field, they are in stable equilibrium in a 

 direction at right angles to the force of the field. 



This is exactly the case of magnetic needles used for certain 

 galvanometers. The system is so much the more nearly astatic the 

 more nearly the direction of the free needles is to being perpen- 

 dicular to the magnetic meridian. 



300. MAGNETIC POLARITY. RUPTURE OF A MAGNET. When 

 a magnetic needle is broken, each of the portions becomes a com- 

 plete magnet having two equal poles of opposite kinds, and the 

 phenomenon can be repeated indefinitely as far as we can carry the 

 division by mechanical means. This is a fact of prime importance 

 in the theory of magnetism : it proves in the first place that it is 

 impossible to get an independent mass of negative or positive 

 magnetism which is not associated with an equal mass of the 

 opposite kind ; and further that magnetism is an essentially molecular 

 phenomenon. We are led to admit that magnetism is due to a 

 kind of polarization of ponderable molecules, each of which is a 

 small magnet with its two poles exactly on the terminal faces. 



301. INDUCED MAGNETISATION. The tufts of iron filings which 

 remain adhering to a magnet prove that each particle of filing has 

 itself become converted into a small magnet. The number of grains 

 in direct contact with the magnet is relatively very small ; the others, 

 attached in succession to each other and to the first, form chains 

 where the particles are united by their poles of contrary names. 

 The magnetisation acquired by these filings is transient; as soon 

 as they are detached from the magnet, they resume their original 

 neutrality. In like manner, a bar of soft iron is magnetised when 

 placed in the prolongation of a magnet, and acquires two poles 

 similarly placed to those of the magnet that is to say, the two 

 adjacent ends of the magnet and the soft iron have magnetisms 

 of opposite signs. This magnetised soft iron may in turn act 

 similarly on a second piece, and so on. As soon as the original 

 magnet is removed-, the magnetisation of the first bar of soft iron 

 and of all those which follow it disappears more or less, and all the 



