INTERACTIONS OF MOVING ELECTRONS 197 



k.e., in moving a small distance like pp', just equals 

 the increase in p.e. Any such outward motion as 

 that represented in the figure would then be impossible. 

 On the other hand, even if not supported, it could not 

 fall toward the earth. Under this condition the kinetic 

 energy of the rotating body would have made unavailable 

 the potential energy which it has by virtue of its position. 



Below this critical speed, that is, under actual 

 conditions, some of the gravitational potential energy 

 will be unavailable. Such a reduction of available 

 potential energy should be most pronounced at the 

 equator, where the surface speed is greatest. Hence 

 "g, " the space rate of change of the available poten- 

 tial energy per gram, should be less l nearer the 

 equator, as experiment shows it to be. 



Suppose, however, that we had been born on an earth 

 which was not revolving and that we had calibrated a 

 number of spring balances after the manner described 

 in Chapter III. Suppose that one night our hypo- 

 thetical earth was set into a rotation like that which we 

 now experience. All of our spring balances would 

 register a little light. If then somebody told us that 

 our earth was revolving we might say that such a rota- 

 tion had resulted in a repulsion being exerted between 

 the earth and all bodies on its surface. 



In much the same way men became accustomed to 

 electricity at rest, but following Volta's work they awoke 

 suddenly to a world hi which electricity is also moving. 



then not show the phenomenon of weight at all. If it should re- 

 volve faster they would fly off into space. 



1 This change in "g" is in addition to the smaller change due to 

 the increased separation, as discussed in Chapter XII. 



