JAMES PRESCOTT JOULE. 847 



electro-magnetic engines, and his description of the mechanism and of 

 its performance, as given in Sturgeon's " Annals of Electricity " m 

 1838 and after, show the definiteness of his ideas and of his work. 

 For instance he tells how many grains his electro-magnets weighed, 

 how many yards of silk^covered copper wire one fortieth of an inch 

 in diameter were wound upon them, how many grains each magnet 

 would hold up, how many revolutions per minute his engine made 

 when supplied with electricity by one or more cells of battery, and 

 how many pounds it was capable of raising one foot in a mmute. 

 Such painstaking to know the value and importance of every step in 

 an inqiury is rare to-day, but it was much rarer fifty years ago ; and 

 to be found in a youth of twenty years, having no supervision and 

 no incentive beyond his own impulses, is surprising indeed. 



It is to be borne in mind, that in those days the so-called various 

 forces, heat, light, electricity, magnetism, and chemism, were supposed 

 to be iudependeut forces ; the idea of correlation among them had not 

 dawned among physicists ; much less had they any conception of what 

 we call to-day the conservation of energy, which implies that all the 

 various forms of energy are not only transformable into one another, 

 but that they are so related q-iantitatively that the appearance of a 

 definite quantity of one kind of energy implies the disappearance of 

 an equal quantity of some other kind. Faraday indeed had already 

 shown such a relation between chemism and electricity. Beyond 

 that there was nothing done, and there were no fundamental ideas 

 such as we to-day start with. WTien, therefore, we find young Joule 

 measuring the work done by an electric motor in foot pounds, and 

 comparmg the result with the amount of zinc dissolved in his bat- 

 tery, it is easy to see how far ahead of his contemporaries he was. 

 From his experiments with batteries and his electro-magnetic engines 

 in 1839 and 1840, he concludes that galvanic batteries using zinc 

 cannot be made to compete with steam-engines using coal as sources 

 of power. His conclusions still hold good after the lapse of fifty 

 years, and the experience of a multitude of experimenters, some of 

 whom continue to work and hope that the laws involved are not so 

 rigorous as Joule supposed. 



In 1841 he discovered that a bar of iron was elongated when it 

 was magnetized, and he measured the amount of the elongation, finding 

 it to be the one thirty-thousandth part of an inch in a bar two feet 

 long when magnetized with the current from five Grove cells. 



In 1840 he presented a paper to the Royal Society containincj an 

 investigation into the distribution of heat in an electric circuit, wherein 



