H. Wilde, J. Hopkinson, J. A. Ewing 193 



John Hopkinson (1849-1898) was born in Manchester, 

 and after studying two years at Owens College entered 

 Trinity College, Cambridge. He graduated in 1871 as senior 

 wrangler, and in the following year was engaged by Messrs. 

 Chance Brothers, glass manufacturers, at Birmingham, as 

 engineering manager. In this position he devoted himself 

 to the improvement of lighthouse illumination, and intro- 

 duced the system of group flashing lights which is now 

 extensively used. In 1878 he settled in London as consulting 

 engineer, and during the next few years conducted his 

 classical researches on the efficiency of dynamo-machines. 

 These were completed later, in conjunction with his brother 

 Edward, by laying down the general principles by which the 

 performance of any machine may be predicted from its 

 design. Another important contribution to electric lighting 

 was his invention of the three-wire system of electrical 

 distribution. 



The efficient working of most of our electrical machinery 

 depends on the magnetic properties of iron, and mention 

 must, therefore, here be made of the valuable investigations 

 of Professor J. A. Ewing, who first clearly pointed out the 

 inevitable dissipation of energy which occurs when a piece of 

 iron is subject to rapidly alternating magnetic forces, as it is, 

 for instance, in a transformer. Owing to a property of iron 

 which he called hysteresis, and which is a kind of internal 

 viscosity brought into action by the rapidly changing orienta- 

 tion of magnetic molecules, some of the energy will always be 

 converted into heat, and is lost as useful work. In other 

 respects also, Ewing has added much to our knowledge of 

 magnetism. 



Our electrical industry owes much to William Edward 

 Ayrton (1847-1908), who was the first to introduce sound 

 methods of instruction in applied electricity. He was 

 the most successful and, for a time, the only teacher of the 

 subject. He organized the laboratories at Finsbury College, 

 and at the Central College, Kensington. Men came from 

 all parts of the world to be trained by him, and he knew how 

 to infuse his students with the spirit of research. In the 

 early days of the industry, the measuring instruments, 

 though suitable for a physical laboratory, could not easily 



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