Albert Einstein 97 



and they are equally rich. Now transfer Bill to New 

 York. Bill is no longer as rich as Jim, because prices in 

 New York are higher than in London. Therefore the 

 pound is not an absolute standard of wealth, but is 

 relative to the place where it is to be spent. 



Simpler still is relativity in direction. If you are in 

 London you say rightly that York lies to the north, but 

 if you are in Edinburgh York is south of you. The direc- 

 tion of every place in the world is relative to your position 

 at the time of speaking. 



Now with some trepidation we will go a little farther. 

 In 1887 two scientists, Michelson and Morley, carried out 

 an experiment by which they proved that speed causes 

 contraction in a moving object. Take a rod moving at a 

 very high speed. At first it is at a right angle to the line 

 of motion, but as it moves we imagine it to be turned so 

 that it lies along the line of motion. In its second position 

 the rod is shorter than it was in its former. The speed of 

 our planet in its journey around the sun is nineteen 

 miles a second, and a rod travelling at this speed contracts 

 one part in two hundred millions. 



This seems so small a matter as to be hardly worth 

 notice, for, applied to the earth itself, it means a con- 

 traction in its diameter of only two and a half inches ; yet 

 we may say that it is on the base of the Michelson-Morley 

 experiment that Einstein has built up his tremendous and 

 revolutionary Relativity and Quantum theories. 



The experiment has been repeated by several observers 

 since 1887, with great care and accuracy, and the con- 

 traction of moving bodies, now known as the Fitzgerald 

 Contraction, is of enormous importance in modern 

 physics. 



The substance of our rod matters not at all. It may be 

 wood or steel or lead. Whatever its substance its con- 

 traction is the same. Each time that you change the 

 position of a foot-rule by holding it in line with the earth's 



