THE FITZGERALD CONTRACTION 5 



was a tribute to the comprehensiveness of Newton's 

 original outlook. 



We have now to see how the classical scheme broke 

 down. 



The FitzGerald Contraction. We can best start from 

 the following fact. Suppose that you have a rod moving 

 at very high speed. Let it first be pointing transverse 

 to its line of motion. Now turn it through a right angle 

 so that it is along the line of motion. The rod contracts. 

 It is shorter when it is along the line of motion than 

 when it is across the line of motion. 



This contraction, known as the FitzGerald contrac- 

 tion, is exceedingly small in all ordinary circumstances. 

 It does not depend at all on the material of the rod but 

 only on the speed. For example, if the speed is 19 miles 

 a second — the speed of the earth round the sun — the 

 contraction of length is 1 part in 200,000,000, or 2^4 

 inches in the diameter of the earth. 



This is demonstrated by a number of experiments of 

 different kinds of which the earliest and best known is 

 the Michelson-Morley experiment first performed in 

 1887, repeated more accurately by Morley and Miller 

 in 1905, and again by several observers within the last 

 year or two. I am not going to describe these experi- 

 ments except to mention that the convenient way of 

 giving your rod a large velocity is to carry it on the 

 earth which moves at high^ speed round the sun. Nor 

 shall I discuss here how complete is the proof afforded 

 by these experiments. It is much more important that 

 you should realise that the contraction is just what would 

 be expected from our current knowledge of a material 

 rod. 



You are surprised that the dimensions of a moving, 



