or dimensions, they all have certain regions, generally two, 

 where their attracting property is most developed, and some 

 intermediate parts where that property is nil. The regions of 

 activity are generally at the extremities of the magnet, and are 

 called poles; whereas the non-active part lies in the middle, 

 and is called neutral zone or neutral line. When there exist 

 more than two active regions, those at the extremities of the 

 magnet retain the name of poles, and the others are called 

 consequent poles. In a closed magnet the poles are situated at 

 the extremities of a same diameter and the neutral zones are 

 situated at the extremities of a diameter perpendicular to the 

 latter. 



When studying the magnets, Ampere came to the con- 

 clusion that each molecule of a magnetic body was a magnet 

 in itself, having two poles and a particular direction, the whole 

 being constituted in such a fashion that the total of the actions 

 on any external point is nil. Under the influence of an external 

 action the molecules set themselves in a special direction, in 

 which they remain in steel and in all bodies possessed of a 

 coercive force. In soft iron, the effects cease with the causes 

 which produce them. According to Ampere's theory, the 

 magnetisation of molecules is the result of permanent currents 

 circulating round each of them. 



When a steel needle is suspended by its centre, it is in 

 indifferent equilibrium ; but if it is magnetised, it will take up 

 a determined position, to which it will revert whenever turned 

 from it. This particular direction is the magnetic meridian, 

 which differs very little from the terrestrial meridian. 



The extremity of the needle pointing towards the north is 

 called North Pole, whereas the other extremity, pointing to- 

 wards the south of the earth, is called South Pole. 



A magnetised bar exerts, in its neighbourhood, a very 

 peculiar action; for example, if a suspended steel or iron 

 needle is brought within a certain distance of it, the said 

 needle will set in a well-determined position, according to its 

 distance from the poles or from the neutral zone. The oscilla- 

 tions described by the needle before setting in an equilibrium 

 position are indicative of the power of attraction in each point ; 



