PRELIMINARY NOTIONS. 3 



when they are short and quick, this power is intense; when 

 they are long and slow, the power is feeble. 



The magnetic field of a magnet is the space surrounding it, 

 and submitted to or pervaded by its magnetic influence. As 

 long as the needle above referred to is not in a state of equi- 

 librium it is in the magnetic field of the magnet. 



In order to study the magnetic properties of a magnet, the 

 magnet may be placed under a sheet of cardboard or of glass, 

 and iron filings spread from a sieve placed at a certain distance 

 above it. In their fall the particles of iron arrange themselves 

 according to well-defined directions, and in masses the more 

 compact the nearer they are situated to the poles. The figures 

 thus formed have been called magnetic phantoms. The curves 

 formed by the particles of iron are called lines of force. Faraday 

 has demonstrated that : (1) a line of force has always a tendency 

 to be as short as possible ; (2) that two parallel lines of force 

 of the same intensity repel each other ; (3) that the number of 

 lines of force passing in each point is in proportion to the mag- 

 nitude of the force in this particular point. 



When two magnets are near each other, their lines of force 

 exert on each other actions which, according to Faraday's law, 

 alter their directions and respective intensities. 



Coulomb has experimentally established that the magnetic 

 attractions and repulsions exerted between two poles are inversely 

 proportional to the square of the distance between them.* 



ENERGY. The energy of any given system is the total 

 quantity of motive force which it possesses or can develop ; in 

 other words, it is the total amount of work which this system is 

 capable of producing.! 



The mechanical work resulting from the fall of a body ; the 

 calorific work due to variations of temperature ; the chemical 

 work which takes place in every transformation of substances ; 

 the electrical work obtained from the reciprocal motion of 

 conductors, &c., are no other than special forms of energy. To 



* The scope of this book does not allow us to enter more in detail into the 

 analysis of magnetic phenomena. See for particulars ' Sur la theorie de la 

 machine Gramme,' by A. Bre'guet (Gauthier-Villars, 1880). 



t E. E. Blavier, ' Des Grandeurs electriques.' 



