PHYSICS 61 



gard it as important, not only because of its theoretical value, 

 but also because in our everyday life this concept of energy 

 is thrust upon us. Energy has a market value, is bought and 

 sold in almost every community. As an example of this take 

 the case of the electric lighting of our homes. The process 

 is somewhat as follows : Coal is burned ; energy is taken from 

 the coal and given to the water and steam in the boilers. By 

 means of the steam engine, this energy now becomes ordinary 

 mechanical energy and is transmitted by shaft or belts to the 

 dynamo. There the energy is transformed into electrical 

 energy. This electrical energy is transmitted along wires into 

 a transformer ; is there changed into magnetic energy ; and 

 that in turn in another circuit develops the electrical energy 

 which comes into our house and in the lamp is changed into 

 still another form of energy — light. None of the electrical 

 current that passed through the lamp came from the distant 

 power house. The only thing that starts from the power 

 house and finally reaches the lamp is energy. Moreover, when 

 we pay our bills, we pay not for light but literally for the 

 energy that has been delivered. For the electric meters are 

 instruments designed for measuring energy and nothing but 

 energy. This example can be duplicated time and time again 

 from almost every field of physical science. It is this uni- 

 versality that compels us to regard energy as one of our funda- 

 mentals. 



So commonly do we meet this term that we sometimes 

 unconsciously drift into the habit of regarding energy as a 

 thing or substance. This shows itself in the attempt of soma 

 to build up a logical system of physics having as the funda- 

 mental concepts, energy, space, time, and mass. 



But lest you think that because this science is built on 

 definitions and conventions the whole subject is conventional, 

 let me remind you that I am trying to give an account of 

 the attitude and the method of the physicist rather than a 

 statement of the extent of his work. He really deals with 



