192 Britain's Heritage of Science 



efficiency of electromagnetic engines. Between 1850 and 

 1860 many attempts were made to increase the intensity of 

 electric currents obtained by electrodynamic induction, but 

 the turning point came when, in the spring of 1867, Henry 

 Wilde, of Manchester, showed some remarkable experiments 

 in the rooms of the Royal Society. In the previous year he 

 had already described the main principle on which he relied 

 to increase the intensity of currents that could be obtained 

 by electromagnetic induction. A machine constructed accord- 

 ing to a model made by Werner Siemens, in which an armature 

 rotated in a magnetic field produced by a permanent magnet, 

 generated an electric current which fed a second and larger 

 machine in which the permanent magnets were replaced by 

 electromagnets. These were excited by the first current 

 and a much stronger magnetic field was produced : a more 

 powerful current was consequently obtained. This was led 

 in a third machine round still larger masses of iron, which 

 were thus magnetized, and finally a current emerged showing 

 effects of surprising intensity. A piece of iron half-an-inch 

 thick melted and burned when the current was made to pass 

 through it, and a rod of platinum two feet long and a quarter 

 of an inch in diameter was also seen to melt. A steam engine 

 of 15 h.p. was required to drive the shafts of the machines. 

 Eye-witnesses testify to the great impression created by 

 these experiments, and there can be little doubt that the 

 public then first began to recognize the potentialities of the 

 electric current. Rapid advances were quickly made, and 

 the modern " dynamo-machine " was soon evolved; Wilde 

 himself had already called his machines by that name. 



As soon as commercial interests are involved in scientific 

 appliances, new problems of an economic nature arise. The 

 weight of metal to be put into the different parts of the 

 machinery has to be adjusted so as to obtain the best 

 results at the least cost, and other matters have to be con- 

 sidered. Apart from some contributions by Lord Kelvin, it 

 may be said that the economics of the dynamo-machine 

 depend almost entirely on the researches of John Hopkinson, 

 who, perhaps, more than any other British man of science, 

 combined the commercial faculty with the highest scientific 

 attainments. 



