Relations between Electrical Measurements. 441 



acts on a free pole ; but this force, /, is also proportional to the 

 strength-, m, of the pole introduced into the field, and it depends 

 on no other quantities ; hence 



f=mU, . (2) 



and therefore the field of unit intensity will be that which acts 

 with unit force on the unit pole. 



The dimensions of H are —r— • 

 L 2 T 



The lines of force produced by a long thin bar magnet near 

 its poles will radiate from the poles, and the intensity of the 

 field will be equal to the quotient of the strength of the pole 

 divided by the square of the distance from the pole; thus the 

 unit field will be produced at the unit distance from the unit 

 pole. In a uniform magnetic field the lines of force, as may be 

 demonstrated, will be parallel; such a field can only be pro- 

 duced by special combinations of magnets, but a small field at a 

 great distance from any one pole will be sensibly uniform. Thus', 

 in any room unaffected by the neighbourhood of iron or mag- 

 nets, the magnetic field due to the earth will be sensibly uni- 

 form ; its direction will be that assumed by the dipping-needle. 



7. Magnetic Moment. — In reality we can never have a single 

 pole entirely free or disconnected from its opposite pole; and it 

 is time to pass to the consideration of the effect produced on a 

 material bar magnet in a magnetic field. In a uniform field 

 two equal opposite and parallel forces act on its poles, and tend 

 to set it with the line joining those poles in the direction of the 

 force of the field. When the magnet is so placed that the line 

 joining the poles is at right angles to the lines of force in the 

 field, this tendency to turn or " couple," G, is proportional to the 

 intensity of the field, H, the strength of the poles, m, and the 

 distance between them, /; or 



G=mM (3) 



ml, or the product of the strength of the poles into the length 

 between them, is called the magnetic moment of the magnet; 

 and from equation (3) it follows, that, in a field of unit intensity, 

 the couple actually experienced by any magnet in the above 

 position measures its moment. The dimensions of the unit of 



magnetic moment are evidently — ^ — • 



8. Intensity of Magnetization. — The intensity of magnetiza- 

 tion of a magnet may be measured by its magnetic moment 

 divided by its volume. 



Phil. Mag. S. 4. Vol. 29. No. 198. June 1865. 2 G 



