THE PHENOMENA OF ELECTROSTATICS. 221 



Similarly, it is sufficient for the science of electro -mechanics 

 that the physical actions which underlie electromotive force, elec- 

 tric current, magnetic field and electric field are what may be 

 called states of permanency ; thus, to say that a current of ten 

 amperes flows through a wire is to specify a definite effect of a 

 definite condition or state of the wire, and it is the correlation 

 between the definite condition and the definite effect that is im- 

 portant rather than the details of the physical action which is 

 taking place in the wire. In fact, the science of electro-mechanics 

 owes its existence to the legitimacy and usefulness of the ideas of 

 electromotive force, electric current, magnetic field and electric field, 

 irrespective of the nature of the physical actions upon which these 

 various things depend. 



The superficial character * of the science of simple mechanics and 

 of the science of electro-mechanics may be further exemplified as 

 follows : Let us consider, on the one hand, the idea of tensile 

 strength. A piece of steel is broken by a tension of 120,000 

 pounds per square inch, but the exact character of the action 

 which takes place in the steel and which constitutes the tension 

 of the steel, and the exact character of the physical action which 

 takes place in the . engine or motor which operates the testing 

 machine and subjects the rod of steel to tension are not matters 

 for consideration. Indeed, nothing at all is known fundamentally 

 as to the physical action which constitutes the tension of a bar of 

 steel. Let us consider, on the other hand, the idea of dielectric 

 strength. A plate of glass is broken down by an electric field of 

 95,000 volts per centimeter, but the exact nature of the stress 

 which constitutes the electric field and the exact character of the 



* What has been said above concerning the scope of mechanics may be exemplified 

 as follows : Simple mechanics is concerned with the correlation of measurable effects, 

 such as the relationship between the size of a beam and the load it can carry, the size 

 of a fly-wheel and the work it can do when it is stopped, the thickness and diameter 

 of a boiler shell and the pressure which it can withstand, the size of a submerged body 

 and the buoyant force which acts upon it, the size and shape of an air column and its 

 number of vibrations per second, and so on. It is evident that such relations as these 

 do not involve any consideration of the intimate nature of the physical actions which 

 are taking place. 



