146 HENRY A. KOWLAND 



again compared with the standard at Washington. Unlike many Ger- 

 man instruments, quite fine wire has always been used and the number 

 of coils multiplied, for in this way the constants of the coils can be 

 more exactly determined, there is less relative action from the wire 

 connecting the coils, and above all we know exactly where the current 

 passes. 



The experiment was performed in the back room of a small house 

 near the University, which was reasonably free from magnetic and other 

 physical disturbances. As the magnetic disturbance was eliminated 

 in the experiment, it was not necessary to select a region entirely free 

 from such disturbance. The small probable error proves that sufficient 

 precaution was taken in this respect. 



The result of the experiment that the British Association unit is too 

 great by about -88 per cent, agrees well with Joule's experiment on the 

 heat generated in a wire by a current, and makes the mechanical equiv- 

 alent as thus obtained very nearly that which he found from friction: 

 it is intermediate between the result of Lorenz and the British Asso- 

 ciation Committee; and it agrees almost exactly with the British Asso- 

 ciation Committee's experiments, if we accept the correction which I 

 have applied below. 



The difference of nearly three per cent which remains between my 

 result and that of Kohlrausch is difficult to explain, but it is thought 

 that something has been done in this direction in the criticism of his 

 method and results which are entered into below. My value, when 

 introduced into Thomson's and Maxwell's values of the ratio of the 

 electromagnetic to the electrostatic units of electricity, caused a yet 

 further deviation from its value as given in Maxwell's electromagnetic 

 theory of light: but experiments on this ratio have not yet attained 

 the highest accuracy. 



HISTORY 



The first determination of the resistance of a wire in absolute meas- 

 ure was made by Kirchhoff 2 in 1849 in answer to a question propounded 

 by Neumann, in whose theory of electrodynamic induction a constant 

 appeared whose numerical value was unknown until that time. His 

 method, like that of this paper, depended on induction from currents: 

 only one galvanometer was used and the primary current was measured 

 by allowing only a small proportion of it to pass through the galvano- 



2 Bestimmung der Constanten von welcher die Intensitat inducirter elektrischer 

 Strome abhangt. Fogg. Ann., Bd. 76, S. 412. 



