a 



350 RADIATION OF ENERGY. 



The difference between the conception of the energy of a moving body and the 

 conception of electrical energy according to Maxwell's theory may be shown bv 

 an illustration. If a pipe through which a steady current of water is flowing is 

 buried in a wall, unless the water makes a noise in flowing, we have no means at 

 present known to us of detecting its existence. On the other hand, if a wire 

 carrying a steady electric current is buried in the wall the existence of the current 

 can be detected, at any point within a reasonable distance, by the behavior of 

 magnet. From this fact comes the assumption that the condition of the space 

 surrounding the wire is that which produces the energy of the current in the wire. 

 The question which I shall consider in this paper is: What do we know 

 about radiant energy and the way that it disappears at one point and appears 

 at another? In the case of the radiation of waves of sound from a material body, 

 such as the sounding board of a piano, we know that the motion of the sounding 

 board is communicated to the surrounding air and travels out in all directions 

 according to well-known mechanical laws. What we want to know is: How do 

 the waves of light, heat, and electricity travel in space, why are they given off by 

 one oscillator and received by another, and how do they go from one point to 

 another? 



§2. On the Conception of Energy. 



The belief in the real existence of energy seems to be almost the only thing 

 which has emerged from the recent discussions of physical principles unchanged, 

 and yet the conception of energy is after all essentially a mathematical con- 

 ception. The energy of a body, or of a system of bodies, measures its power of 

 doing work. Mechanical energy is expressed either as one half of mass times 



velocity squared, or as force times distance. It is not in any sense a primitive 

 or fundamental conception. The savage is familiar with the ideas of time, 

 distance and force, but has no conception of energy. Indeed we have only to 

 go back three centuries to find a hopeless confusion of the ideas of force and 

 energy. Energy is a concept of the highly trained mathematical mind. It is 

 a mathematical function of the same kind as entropy 1 and action . To state the 

 principle of the conservation of energy is to state a mathematical relation govern- 

 ing physical phenomena. 



Mach says: "Energy and entropy are metrical notions." 2 Also, "If we esti- 

 mate every change of physical condition by the mechanical work which can be 

 performed upon the disappearance of that condition, and call this measure energy 

 then we can measure all physical changes of condition, no matter how different 

 they may be with the same common measure , and say : the sum total of all energy 

 remains constant."* 



a statement of the parallelism between energy and entropy see Planck, Physikalisehe ZeUaehnfl, 

 aug 13, pp. 168, 169, 1912. r- rt Pub- 



On the Principle of the Conservation of Energy, Popular Scientific Lectures, Open Court ru 



> 



Jahrgang 



lishing 



Ibid., d. 164 



