156 REPORT—~1863. 
ductor. He showed how to measure it, though not in absolute measure, by 
his proof-plane. 
« Resultant Electric Force—The resultant force in air or other insulating 
fluid in the neighbourhood of an electrified body is the force which a unit of 
electricity concentrated at that point would experience if it exercised no 
influence on the electric distributions in the neighbourhood. The resultant 
force at any point in the air close to the surface of a conductor is perpen- 
dicular to the surface, and equal to 4 zp, if p designates the electric density of 
the surface in the neighbourhood. 
“ Electric Pressure from the Surface of a Conductor balanced by Air.—A 
thin metallic shell or liquid film, as for instance a soap-bubble if electrified, 
experiences a real mechanical force in a direction perpendicular to the sur- 
face outwards, equal in amount per unit of area to 27p*, p denoting as before 
the electric density at the part of the surface considered. In the case of a 
soap-bubble its effect will be to cause a slight enlargement of the bubble on 
electrification with either vitreous or resinous electricity, and a corresponding 
collapse on being perfectly discharged. In every case we may consider it as 
constituting a deduction from the amount of air-pressure which the body ex- 
periences when unelectrified. The amount of deduction being different at 
different parts according to the square of the electric density, its resultant 
action on the whole body disturbs its equilibrium, and constitutes in fact the 
resultant electric force experienced by the body.” 
49, Tension.—The use of this word has been intentionally avoided by us 
in this treatise, because the term has been somewhat loosely used by various 
writers, sometimes apparently expressing what we have called the density, 
and at others diminution of air-pressure. By the most accurate writers it 
has been used in the sense of a magnitude proportional to potential or differ- 
ence of potentials, but without the conception of absolute measurement, or 
without reference to the idea of work essential in the conception of potential. 
We believe also that it has not been generally, if ever, applied to that con- 
dition of an insulating fluid in virtue of which each point has an electric 
potential, although no sensible quantity of electricity be present at the point. 
The expression ‘‘ tension” might be used to designate what we have termed 
the potential of a body. The tension between two points would then be 
equivalent to the electromotive force between those points, or to their differ- 
ence of potentials, and would be measured in the same unit. 
50. Conducting Power, Specific Resistance, and Specific Conducting Power. 
Conducting Power, or Conductivity.—These expressions are employed to sig- 
nify the reciprocal of the resistance of any conductor. Thus, if the resistance 
of a wire be expressed by the number 2, its conducting power will be 0-5. 
Specific Resistance referred to unit of Mass—The specific resistance of a 
material at a given temperature may be defined as the resistance of the unit 
mass formed into a conductor of unit length and of uniform section. Thus 
the specific resistance of a metal in the metrical system is the resistance of a 
wire of that metal, one metre long, and weighing one gramme. 
The Specific Conducting Power of a material is the reciprocal of its specific 
resistance. 
Specific resistance, referred to unit of volume, is the resistance opposed by 
the unit cube of the material to the passage of electricity between two opposed 
faces. It may easily be deduced from the specific resistance referred to unit 
of mass, when the specific gravity of the material is known. 
Specific Conducting Power may also be referred to unit of volume. It is of 
course the reciprocal of the specific resistance referred to the same unit, 
