THE FITZGERALD CONTRACTION 7 



an entirely natural property of a swarm of particles held 

 in delicate balance by electromagnetic forces, and occu- 

 pying space by buffeting away anything that tries to 

 enter. Or you may look at it this way: your expecta- 

 tion that the rod will keep its original length presup- 

 poses, of course, that it receives fair treatment and 

 is not subjected to any new stresses. But a rod in motion 

 is subjected to a new magnetic stress, arising not from 

 unfair outside tampering but as a necessary consequence 

 of its own electrical constitution; and under this stress 

 the contraction occurs. Perhaps you will think that if 

 the rod were rigid enough it might be able to resist the 

 compressing force. That is not so; the FitzGerald con- 

 traction is the same for a rod of steel and for a rod of 

 india-rubber; the rigidity and the compressing stress are 

 bound up with the constitution in such a way that if 

 one is large so also is the other. It is necessary to rid 

 our minds of the idea that this failure to keep a constant 

 length is an imperfection of the rod; it is only imperfect 

 as compared with an imaginary "something" which has 

 not this electrical constitution — and therefore is not 

 material at all. The FitzGerald contraction is not an 

 imperfection but a fixed and characteristic property of 

 matter, like inertia. 



We have here drawn a qualitative inference from the 

 electrical structure of matter; we must leave it to the 

 mathematician to calculate the quantitative effect. The 

 problem was worked out by Lorentz and Larmor about 

 1900. They calculated the change in the average spacing 

 of the particles required to restore the balance after it 

 had been upset by the new forces due to the change of 

 motion of the charges. This calculation was found to 

 give precisely the FitzGerald contraction, i.e. the amount 

 already inferred from the experiments above mentioned. 



