184 REPORT—1863. 
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 produced 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 neigh- 
bourhood of iron or magnets, the magnetic field due to the earth will be 
sensibly uniform ; 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 con- 
sideration 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, mm, 
and the distance between them, 7; or 
Gomis os (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 
L? M? 
1 
8. Intensity of Magnetization.—The intensity of magnetization of a magnet 
may be measured by its magnetic moment divided by its volume. 
unit of magnetic moment are evidently 
The dimensions of the unit of magnetization are therefore rt 
the same as in the case of intensity of field. 
9. Coefficient of Magnetic Induction—When certain bodies, such as soft 
iron, &c., are placed in the magnetic field, they become magnetized by “ induc-’ 
tion”; so that the intensity of magnetization is (except when great) nearly 
proportional to the intensity of the field. 
In diamagnetic bodies, such as bismuth, the direction of magnetization is 
opposite to that of the field. In paramagnetic bodies, such as iron, nickel, &c., 
the direction of magnetization is the same as that of the field. 
The coefficient of magnetic induction is the ratio of the intensity of mag- 
netization to the intensity of the field, and is therefore a nwmerical quantity, 
positive for paramagnetic bodies, negative for diamagnetic bodies. 
10. Magnetic Potentials and Equipotential Surfaces—If we take a very 
long magnet, and, keeping one pole well out of the way, move the other pole 
from one point to another of the magnetic field, we shall find that the forces 
in the field do work on the pole, or that they act as a resistance to its motion, 
according as the motion is with or contrary to the force acting on the pole. 
If the pole moves at right angles to the force, no work is done. 
The magnetic potential at any point in a magnetic field is measured by the 
work done by the magnetic forces on a unit pole during its motion from an 
infinite distance from the magnet producing the field to the point in question, 
supposing the unit pole to exercise no influence on the magnetic field in 
question. The idea of potential as a mathematical quantity having different 
