STANDARD GALVANOMETER. ^7 



that the results he had obtained had led him to make a 

 new departure which must be regarded as a fundamental 

 step in attaining the ultimate success of his research. So 

 far he had made no electrical measurements; he had com- 

 pared his different magnets byconnecting them with the same 

 battery, the only other measurements being as to the effects 

 on the same magnet of varying the number of cells and the 

 charge. His fourth letter, May the 14th, 1839, commences : 

 "It was important to use in the research a galvanometer, the 

 indications of which could be relied upon." This letter is 

 written less than two months after the previous one, yet in 

 all the account of his previous 14 months' work there is no 

 suggestion of even the desirability of measuring the currents 

 of his batteries. In his first two letters he ends by indicating 

 the line in which he intends to proceed ; it would therefore 

 seem probable that he had only realized the importance of 

 this step since the 27th of March, 1839. In his fourth letter 

 he proceeds at once to the description of the galvanometer 

 which is an instrument of his own device and construction, 

 far in advance of anything of the kind existing at that time, 

 and for its purpose as perfect as it would now be possible to 

 make it. That he was fully acquainted with the principles 

 of its action, and had thought deeply as to its construction, 

 is abundantly evident. In this letter, after describing the 

 instrument, he gives no indication as to what is his intention 

 in using it, but proceeds to describe experiments with the 

 magnets previously constructed in which the current is 

 measured with the galvanometer. 



The result of these experiments is to reveal an important 

 law up to that time unknown, " that the attractive force of 

 two electro-magnets for one another is as the square of the 

 product of the current and the length of the wire" 



