252 Professor Fleming [June 5, 



then drop it over the coil the lamp for one instant goes out, but it 

 brightens up again as the metal cap instantly warms up. This 

 shows us, however, that if the cap were at the absolute zero of 

 temperature it would then be a complete screen for the induction. 

 In fact, these experiments furnish us with a new definition of what 

 we mean by the absolute zero of temperature. It is the tempera- 

 ture at which perfectly pure metals cease to have any electrical 

 resistance. 



In the conduction of currents at ordinary temperatures as we 

 generally know it, two effects are inseparably connected with the 

 conveyance of energy by this process. One is the dissipation of some 

 of the energy as heat in the conductor, the other is a loss of potential 

 or fall of electric pressure, the latter being one of the factors in the 

 equivalent of the energy so dissipated. If, however, the conductor is 

 at the absolute zero of temperature, there would be no heat produced 

 in it, and no fall of potential along it, either for large or small 

 currents. What then under these conditions is the function of the 

 conductor ? The answer is, that it becomes a mere boundary serving 

 to limit the electro-magnetic field and determine the direction in 

 which the energy transmission is taking place. These experiments 

 therefore may be regarded as forging one more important link in that 

 chain of experimental evidence which compels us to look for the 

 processes concerned in the conveyance of energy by an electric 

 current, not inside the conductor as we call it, but in the dielectric 

 or medium outside. We may then ask. How is it that different bodies 

 have such various dissipative powers when acting in this way as the 

 boundary of an electro-magnetic field ? The only suggestion on this 

 point I venture to make here is as follows : — Materials of high 

 specific resistance have all probably a very complex molecular struc- 

 ture. The alloys of high resistivity are probably not merely soli- 

 dified mechanical mixtures of metals, but chemical compounds, and 

 even in the case of elementary bodies like carbon and sulphur, which 

 have high resistivity, these last-named bodies may have, owing to 

 their high valency and tendency of their atoms to auto-combination, 

 a complex molecular structure. 



This structure may bestow upon them the power of taking up 

 energy from the electro-magnetic medium, just as gases with a highly 

 complex molecular structure are very absorbent of radiant heat, which, 

 if the electro-magnetic theory of light is true, is only another form 

 of electro-magnetic energy. All we know at present about the pro- 

 cesses at work during the time a conductor is traversed by an electric 

 current, is that there is a magnetic field outside the conductor and 

 also within the mass of the conductor, and that some mechanism is at 

 work absorbing energy through the surface of the conductor and 

 dissipating it as heat in the interior. The resistance of a conductor 

 is best defined as, and numerically measured by, the number express- 

 ing the rate at which it dissij)ates electro-magnetic energy per unit 

 of current. For the same current, that is for the same external 



