24 Britain's Heritage of Science 



results in a series of letters addressed to Sturgeon's 

 " Annals of Electricity," and in the fourth of them he 

 gives us the guiding motive of his research. 



" I can hardly doubt," he writes, " that electro- 

 magnetism will ultimately be substituted for steam to 

 propel machinery. If the power of the engine is in 

 proportion to the attractive force of its magnets, and 

 if this attraction is as the square of the electric force, 

 the economy will be in the direct ratio of the quantity 

 of electricity, and the cost of working the engine may 

 be reduced ad infinitum. It is, however, yet to be deter- 

 mined how far the effects of magnetic electricity may 

 disappoint these expectations." 



Sturgeon's electro-magnetic engine which Joule tried to 

 improve was a very primitive machine. His first attempt 

 to render it more effective was not successful, as he admits ; 

 but what is remarkable is the strictly scientific manner in 

 which he measured the power by the weight the engine 

 could raise per minute. Joule next turned his attention 

 to the measurement of the electric power absorbed. He 

 designed and constructed a galvanometer for the purpose, 

 and as a first result discovered an important law (subse- 

 quently shown to be only approximately true), which appeared 

 to him to justify his belief in the future of the electro -magnetic 

 engine. The passage quoted above in which he expresses 

 this belief shows, however, that consideration of the con- 

 servation of energy had not crossed his mind at that time, 

 and that he considered it possible to have an effective machine 

 the cost of working which may be reduced ad infinitum. 

 He had, nevertheless, some scruples about the effects of 

 " magnetic electricity," which may disappoint his expecta- 

 tions. He therefore directed his attention to these effects. 

 Referring to the impossibility of understanding experiments 

 made by different investigators, " which is partly due to 

 the arbitrary and vague numbers which are made to 

 characterize the electric current," he adopted a system of 

 units which can be reproduced anywhere, using the amount 

 of water decomposed per hour as the standard of current, 

 and the quantity of electricity delivered in one hour by the 

 unit current as the unit quantity. 



