WILHELM WEBER 267 



the method of calculating its results, in a very high 

 degree.^ 



In order to fix the unit current thus realised for all time in 

 a simple manner, Weber, relying on Faraday's first law of 

 electrolysis, made use of the voltameter. The choice of the 

 unit current (now called the ampere) and also the means of 

 fixing it, are still valid to-day. ^ 



In the year 1843 Weber was called to the University of 

 Leipzig, where he then invented his electro-dynamometer, 

 which depended upon Ampere's discovery of the force 

 exerted by one current upon another, and was suited for 

 the finest and most accurate measurement of these effects. 



Six years later Weber again returned to Gottingen, where 

 he remained for the rest of his life. He then began the in- 

 vestigation which led to the settlement of the absolute unit of 

 electrical tension or electromotive force (called to-day the 

 volt), for which Weber relied upon Faraday's law of in- 

 duction, which was possible without the use of the idea of 

 lines of force, since he made use of Gauss' measurement of 

 the earth's magnetic field and the induction produced by it. 

 His earth inductor, with which he made very extensive 

 quantitative measurements, afterwards became one of the 

 most important appliances in electromagnetic measure- 

 ment. 



1 This also allowed a first exact proof of the law, according to which the 

 effect of current of any length and form upon magnetic poles in any posi- 

 tion can be calculated. This law is usually given under the names of 

 Biot and Savart. These scientists only investigated rectilinear currents 

 (see Biot's Elements of Electricity and Magnetism, trans. J. Farrar, Cam- 

 bridge, N.E., 1826); Ampere, took for comparison zig-zag currents, and 

 an observation by Laplace gave the generalisation for 'current elements' 

 (short lengths of current), in any position as compared with the magnetic 

 pole. The circular currents in the tangent galvanometer then gave 

 the best case for an exact test. 



2 The realisation of this in a later and more refined manner, which 

 reached an exactitude not exceeded to-day, was performed in the year 

 1 88 1 (and repeated in 1886), by Friedrich Kohlrausch (1840-19 10, 

 president of the Reichsanstalt in Berlin); he measured the exact 

 amount of silver deposited by one ampfere flowing for one second. 



