108 THE REALITIES OF MODERN SCIENCE 



route which led from a high potential to a low poten- 

 tial. 



Returning to our idea of the gravitational poten- 

 tial of a point as the potential energy of a drop of water 

 at that point, the reader has doubtless felt that the drop 

 of water forms an unsatisfactory unit. With this we 

 agree and suggest that we speak of a pound of water 

 or preferably a gram of water, and then abstracting 

 the water leave merely the mass. The gravitational 

 potential of a point is the potential energy of a gram 

 at the point. 



In case we use grams we should use the centimeter 

 as the unit of distance. The potential of the surface 

 which is one centimeter above sea level we shall call 

 one gram-centimeter (1 g. cm.). The amount of work, 

 then, which one gram can do in falling from a point 

 one centimeter above sea level to sea level is a gram- 

 centimeter. Conversely, it would require one gram- 

 centimeter to lift a gram from sea level to a point one 

 centimeter above it. 



Whenever we pass from a point of lower potential 

 to one of higher potential, work is required, that is, 

 energy must be supplied. Similarly, in passing from 

 a point of higher to one of lower potential, energy is 

 released. When we supply energy we do work, that 

 is, we exert a force through a distance. On the other 

 hand, when energy is released, work is done for us. 

 In both cases a force comes into play when the poten- 

 tial is changed. If the potential is increased a force 

 must be applied to the system. On the other hand, 

 if the potential is decreased the acting force is supplied 

 by the system under consideration. In a later chapter 



