326 Dynamic Theory. 



arate two attracting poles, the position in which they are left, with refer- 

 ence to each other after the exertion of such energy, will be a position 

 of potential energy. In each of these cases the measure of the energy 

 is the potential position multiplied by the force of the magnet. If the 

 fixed magnet possess twice the force it will exert twice the energy, the 

 potential position remaining the same in each case. 



The same principles govern in electricit} r . If a unit of positive elec- 

 tricity be moved near a larger quantity also positive, it will be repelled, 

 and if released the unit quantity will fly off, doing work equal to that re- 

 quired to bring it up. The point to which it is brought is its potential, 

 and it is evident in this case equally with the other cases, that the en- 

 ergy with which it will leave this potential will be in proportion to the 

 quantity of the electricity, two units being repelled with twice the en- 

 ergy with which one. is repelled. When we know the amount of work 

 required to move a body into a potential position, and the force against 

 which it is moved, we can find the value of the potential by the follow- 

 ing formulas : 



Gravitation potential = Work g 



Magnetic potential Workdone in movin g magnetic P oie. 



Strength of pole. 



Electrostatic potential = Work done in Q^^ f electricity ' 

 Potential is therefore measured by work. At infinite distances all at- 

 tractions and repulsions cease. If any of them ceases at a finite dis- 

 tance, that point is a position of a zero potential. The electrostatic po- 

 tential at any point nearer than the zero point is measured by the work 

 required to bring a unit quantity of positive electricity to that point 

 from the zero point. 



Potential energy always tends to unwind and run down to zero; a 

 weight falling to the ground if not prevented, and a magnet pole mov- 

 ing away into a less potential position if it is near another of the same 

 name. In any electrified region the direction in which a unit of posi- 

 tive electricity tends to move shows the direction of lower potential. It 

 will either move away from a positive electricity or towards a negative 

 electricity, and work will have to be done to move it towards the former 

 and to restrain it from going to the latter. 



Induction is due to the following facts : 1st, There is around every 

 magnet, whether a permanent or electro magnet, a field of force, which 

 is constituted of lines of force or electrical motion. About the perma- 

 nent magnet these lines of force curve away from the north pole and 

 make a circuit and return to the south pole. If a bar magnet be covered 

 with a piece of paper and fine iron filings be sifted over it they will be 

 arranged in these lines of force so far as that one plane is concerned, 

 but we are to understand that if the bar were turned on edge and the 



