Fitzgerald — On Limits to Velocity of Motion of Engines. 163 



engine, for the electro-magnetic forces will become greater than the 

 electro-static. The way in which this acts is as follows : — Suppose 

 a charged body, e.g. the carrier in any of the multiplier forms of 

 electro-static engines, move near a conductor, it induces on this latter 

 an electric charge which moves along with the moving carrier. I 

 must neglect the resistance of the conductors, because it being of 

 the nature of friction in ordinary engines limits the velocity in 

 quite a different way from the ways I am considering. Now, if the 

 carrier move with the velocity of light, it and its induced charges 

 will have no action on one another, and so there will be no forces 

 tending to move the carrier. Similarly, if a plate with a charge on 

 it move parallel to a conducting-plate, the moving electrification 

 while its velocity is increasing induces a current in the conducting- 

 plate which is permanent, because the conducting-plate is supposed 

 to be a perfect conductor, and the electro-magnetic action of these 

 two, when the moving-plate moves with the velocity of light, is 

 equal and opposite to their electro-static attraction. Thus it appears 

 that the velocity of light is a limiting velocity to the rate of motion 

 of these engines, just as the velocity of the particles of steam is a 

 limit to the rate of motion of the piston in a steam engine. There 

 is the same limit to the rate of working of electro-magnetic engines. 

 Consider a very simple case. Suppose a wire sliding on two 

 parallel rails with a magnetic force at right angles to their plane, 

 and an electro-motive force driving a current round the circuit. 

 If the magnetic force be feeble enough there seems at first 

 sight no limit to the ultimate velocity of motion of the wire. If 

 we consider, however, what takes place when the electricity goes 

 across from the rails to the moving wire, we see that the reason it 

 goes across is because an electrification on the rails induces a charge 

 on the moving wire, and these attract one another and combine, this 

 action being kept going constantly by the fresh charges supplied by 

 the battery. Now if the wire move with the velocity of light, there 

 will be no longer any action between these charges, and so the wire 

 will act practically as a non-conductor. A conductor moving with 

 the velocity of light acts in other respects as a non-conductor, for it 

 is evident that we can have any desired distribution of electricity 

 in it or on it without any tendency for it to change. It would be 

 more correct to describe it as a region in which the electro-static 



inductive capacity was infinite, and where, consequently, a given 



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