60 THE PHILOSOPHY OF BIOLOGY 



against the constraint of powerful electro-magnets, and 

 in so doing a current of electricity is generated. By 

 the law of conservation this current should contain 

 as much energy as was put into the rotation of the 

 armature ; as a matter of fact it does not, and the 

 deficiency is represented by the friction of the parts 

 of the machine against each other, by imperfect con- 

 ductivity of electricity in the wires, and by imperfect 

 insulation of the current. Friction, imperfect con- 

 ductivity, and imperfect insulation all transform to 

 heat, and this radiates away. Suppose now that the 

 current is used for lighting purposes : to do this it 

 must heat the metallic filaments in the lamps, or the 

 points of the carbons in an arc. This heat then trans- 

 forms to light, but along with the light, which was 

 the object of the transformation, heat is produced, 

 and this heat radiates away. 



The actual process in which the particular form 

 of energy required is generated may or may not be 

 reversible in theory. That employed in the steam- 

 engine is not, for if we start with a cold boiler and then 

 work the engine backwards we could not raise steam. 

 The process in the dynamo is theoretically reversible : 

 if we send a current of electricity into a dynamo the 

 machine will begin to rotate, and become a motor, 

 so that we can obtain mechanical work from it. Now 

 in theory all forms of energy are mutually convertible, 

 and all can be expressed in terms of a common unit. 

 The unit of mechanical energy is called the erg : let 

 a current, the energy of which is equal to N ergs, be 

 sent into the dynamo, then we ought to obtain from 

 the latter mechanical energy equal to A^ ergs. Con- 

 versely, if N ergs of mechanical energy be employed 

 to rotate the dynamo, we should obtain electrical 

 energy equal to this amount. Now as a matter of 



