# J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’

```                                Absolute Values of the
Electromotive and Magnetic Forces called into play in the Propagation of Light

(108) If the equation of propagation of light is
F = A cos 2[pi]/[lambda] (z - Vt)
The electronic force will be
P = - A 2[pi]/[lambda] V sin 2[pi]/[lambda] (z - Vt)
The energy per unit of volume will be
p<sup>2<\sup>/8[pi][mu]V<sup>2<\sup>
Where p represents the greatest value of the electromotive force
Half of this consists of magnetic and half of electric energy.
The energy passing through a unit of area is
W = p<sup>2<\sup>/8[pi][mu]V
so that P = [square root]8[pi][mu]VW
where V is the velocity of light and W is the energy communicated
to unit of area by the light in a second.
According to Pouillets data as calculated by Prof W Thomson*
the mechanical value of direct sunlight at the Earth is
83.4 foot pounds per second per square foot
This give hte maximum value of P in direct sunlight at the
Earths distance from the Sun
P = 60.000.000
or about 600 Daniell's cells per metre.
At the Suns surface the value of P would be about
13000 Daniells cells per meter
At the Earth the maximum magnetic force would be .193+
At the Sun it would be 4.13
These electromotive and magnetic forces must be conceived
to be reversed twice in every vibration of light, that is more
than a thousand million million times in a second.

* Trans Royal Society of Edinburgh 1854 (Mechanical Energies of the Solar System)
+ The horizontal magnetic force at Kew is about 1.76 in metrical units.

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## Manuscript details

Author
James Clerk Maxwell
Reference
PT/72/7
Series
PT
Date
1864
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## Cite as

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