
1904. | The Dual Force of the Dividing Cell. 553 
draw a line such that at every part of its course it is coincident in direction 
with the force at that point, this line may be called a ‘line of force,’ since it 
indicates the direction of the force at every part of its course.”* It will be 
noted that sucha line is a geometrical line, not a material line. Maxwell 
also recognised that for a field of two “unlike” poles the lines have. the 
distribution of stream-lines in liquid in relation to an upwelling source and 
an engulfing sink, and, like Kelvin, saw that this distribution was identical 
with the lines of the flow of heat in a conductor between a source of heat 
and a refrigerator ; while the field of two “like” poles corresponded with 
the thermal field of two sources of heat, or two refrigerators, as the case 
might be. 
A quantitative meaning has been attached to the conception of “ lines of 
force,” and with this addition it now forms the base of the proximate or 
practical theory of electrical engineering. I can best illustrate this 
quantitative connotation by the use of an analogy which I have myself found 
helpful. We may attach such a quantitative meaning to the “ ray of light” 
in geometrical optics, by defining the intensity of illumination of a surface as 
the number of “unit rays” falling on it at right angles (or their equivalent 
at other angles) per unit of area. Adopting this convention, we pass to the 
"question of “ permeability.” 
We find that if the medium traversed by lines of force be not uniform, 
the lines of certain forces travel more readily in some substances than in 
others. Thus magnetic lines pass more readily through soft iron than through 
air, through air than through bismuth; similarly electrostatic force travels 
more readily in sulphur than in air, etc. Such differences are termed 
differences of “permeability ” in relation to magnetism, of “ specific inductive 
capacity ” for electricityt; but the term permeability has already received a 
wide general application ; and we shall use it in relation to mitokinetic force, 
without making any assumption as to its nature. The “permeance” of any 
portion of a field of force is defined as the product of the permeability of the 
* ‘Sci. Works,’ vol. 1, p. 467. 
+ [I find that the term was indeed introduced into Physics in this wide sense by lon 
Kelvin in 1872: “The common word ‘ permeability’ seems well adapted to express the 
specific quality in each of the four analogous subjects. Adopting it we have thermal 
permeability, a synonym for thermal conductivity ; permeability for lines of electric 
force, a synonym for the electro-static inductive capacity of an insulator; magnetic 
permeability, a synonym for conducting power for lines of magnetic force ; and hydro- 
kinetic permeability, a name for the specific quality of a porous solid, according to which, 
when placed in a moving frictionless liquid, it modifies the flow.” A Mathematical 
Theory of Magnetism,” in ‘ Papers on Electrostatics and Magnetism,’ Section 628.—Added 
during the printing. | 
