Part V Theory of Condensers Capacity of a Condenser (3) The simplest form of condenser consists of a uniform layer of insulating matter bounded by two conducting surfaces, and its capacity is measured by the quantity of electricity in either surface when the difference of potentials is unity Let S be the area of either surface, a the thickness of the dielectric and k its coefficient of electric elasticity then on one side of the condenser the potential is [psi]<sub>1<\sub> and on the other side [psi]<sub>1<\sub> + 1 and within its substance d[psi]/dx = 1/a = kf (48) Since d[psi]/dx and therefore f is zero outside the condenser, the quantity of electricity on its first surface =  Sf and on the second + Sf. The capacity of the condenser is therefore Sf = S/ak in electromagnetic measure Specific Capacity of Electric Induction If the dielectric of the condenser be air then its capacity in electrostatic measure is S/4[pi]a (neglecting corrections arising from the conditions to be fulfilled at the edges. If the dielectric have a capacity whose ratio to that of air is D then the capacity of the condenser will be DS/4[pi]a Hense D = k<sub>0<\sub>/k* where k<sub>0<\sub> is the value of k in air which is taken for unity. *49
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Manuscript details
 Author
 James Clerk Maxwell
 Reference
 PT/72/7
 Series
 PT
 Date
 1864
 IIIF

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Cite as
J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7
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