326 COMSTOCK— THE MODERN THEORY [April 22, 



would be stored up throughout the whole mass, a great deal of it 

 near the center, but an appreciable amount even out at the very 

 limits of the block. From the present point of view, we think more 

 about the energy and less about the ether, but the general effect is 

 the same. 



Let us call this energy located in the space surrounding an 

 electric charge " bound energy," to distinguish it from the closely 

 similar type of self-propagating energy which can also exist in 

 space and to which we apply the term " radiant," and let us then 

 ask what properties, if any, this bound energy gives to the body 

 which it surrounds. 



I stated before that radiant energy, when it struck a body, com- 

 municated momentum to it in the same general way as a material 

 body, say a flying bullet, would do. The radiant energy thus acts 

 as if it had mass, and the question now is, "May the 'bound 

 energy ' surrounding our charged sphere also be considered to 

 possess mass?" We may answer this question /;; flic affirmative, 

 for this bound energy is electric energy, and, thanks to the great 

 founders of electrical science, we know the laws of electric action 

 pretty completely. 



Applying these well-known laws we find that when our charged 

 sphere is moved, it acts like a current of elecricity and sets up a 

 magnetic field about it, and the formation of this field acts, by the 

 well-known laws of induction, to retard the motion of the charge. 

 Thus the setting in motion of the sphere is made more difficult by 

 reason of its charge, an eft'ect which is equivalent to saying that 

 the sphere has added mass. 



If this were all, we might say that the added charge of elec- 

 tricity had mass, so that the mass increment is on the surface of 

 the sphere where the charge is known to reside. This is not all, 

 however, for by letting the sphere expand we can decrease the 

 energy in the space about it without changing the magnitude of its 

 charge ; and we find, by further simple application of well-known 

 electrical laws, that the new mass will change exactly as the energy 

 changes. If the new mass had been proportional to the charge, it 

 would remain constant with it instead of changing with the energy. 

 Thus what is known as the electromagnetic mass of the sphere is 



