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

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                                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 
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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James Clerk Maxwell
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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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