588 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



the electromotive force will be 



and 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 



P 1 



W 

 ~ 



so that 



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 Pouillet's data, as calculated by Professor W. Thomson*, the 

 mechanical value of direct sunlight at the Earth is 



8 3 '4 foot-pounds per second per square foot. 



This gives the maximum value of P in direct sunlight at the Earth's distance 



from the Sun, 



P = 60,000,000, 



or about 600 Daniell's cells per metre. 



At the Sun's surface the value of P would be about 



13,000 Daniell's cells per metre. 



At the Earth the maximum magnetic force would be '193t. 

 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. 



* Transactions of MM Royal Society of Edinburgh, 1854 ("Mechanical Energies of the Solar 

 System "> 



t The horizontal magnetic force at Kew is about 1*76 in metrical units. 



