541-543] Dynamical Theory of Currents 475 



The reason why we have been brought to a halt in the development of 

 electromagnetic theory will become clear as soon as we contrast this theory 

 with the theory of gases. The ultimate mechanism with which the theory of 

 gases is concerned is that of molecules in motion, and we know (or at least 

 can provisionally assume that we know) the ultimate laws by which this 

 motion is governed. On the other hand the ultimate mechanism with which 

 electromagnetic theory is concerned is that of action in the ether, and we are 

 in utter ignorance of the ultimate laws which govern action in the ether. 

 We do not know how the ether behaves, and so can make no progress towards 

 explaining electromagnetic phenomena in terms of the behaviour of the ether. 



542. There is a branch of dynamics which attempts to explain the 

 relation between the motions of certain known parts of a mechanism, even 

 when the nature of the remaining parts is completely unknown. We turn to 

 this branch of dynamics for assistance in the present problem. The whole 

 mechanism before us consists of a system of charged conductors, magnets, 

 currents, etc., and of the ether by which all these are connected. Of this 

 mechanism one part (the motion of the material bodies) is known to us, while 

 the remainder (the flow of electric currents, the transmission of action by the 

 ether, etc.) is unknown to us, except indirectly by its effect on the first part 

 of the mechanism. 



543. An analogy, first suggested by Professor Clerk Maxwell, will ex- 

 plain the way in which we are now attacking the problem. 



Imagine that we have a complicated machine in a closed room, the only 

 connection between this machine and the exterior of the room being by 

 means of a number of ropes which hang through holes in the floor into the 

 room beneath. A man who cannot get into the room which contains the 

 machine will have no opportunity of actually inspecting the mechanism, but 

 he can manipulate it to a certain extent by pulling the different ropes. If, 

 on pulling one rope he finds that others are set into motion, he will under- 

 stand that the ropes must be connected by some kind of mechanism above, 

 although he may be unable to discover the exact nature of this mechanism. 



In this analogy, the concealed mechanism is supposed to represent those parts of the 

 universe which do not directly affect our senses e.g. the ether while the ropes represent 

 those parts of which we can observe the motion e.g. material bodies. In nature, there 

 are certain acts which we can perform (analogous to the pulling of certain ropes), and these 

 are invariably followed by certain consequences (analogous to the motion of other ropes), 

 but the ultimate mechanism by which the cause produces the effect is unknown. For 

 instance we can close an electric circuit by pressing a key, and the needle of a distant 

 galvanometer may be set into motion. We infer that there must be some mechanism 

 connecting the two, but the nature of this mechanism is almost completely unknown. 



Suppose now that an observer may handle the ropes, but may not pene- 

 trate into the room above to examine the mechanism to which they are 



