Determination of Resistances in Absolute Measure. 341 



gations upon this subject. It is founded upon II.; but in order 

 to avoid the difficulty arising from the necessity of using a 

 magnet small relatively to the coil in which it is suspended, no 

 attempt is made to determine the constant from the data of 

 construction. The inductor is connected with a sensitive gal- 

 vanometer, and the constant of the latter is deduced from 

 observations of the logarithmic decrement of the vibrations of 

 the magnet when ths circuit is closed (X), and when it is open 

 (\ ). The result, however, involves H the horizontal inten- 

 sity, K the moment of inertia of the needle, as well as the time 

 of vibration T. Expressed roughly, in the notation previously 

 employed, it is 



p _ 32a 4 H 2 TA, AB 



K ' (A 2 + B 2 ) 2 ' 



where B. is the resistance of the circuit composed of the in- 

 ductor and galvanometer, A and B are the arcs of vibration 

 in the method of recoil. 



Interesting as this method is in some respects, I cannot but 

 agree with Rowland in thinking that the final formula is 

 enough to show that it cannot compete with others on equal 

 terms, if the object be to obtain a result of high accuracy. 

 The horizontal intensity itself is perhaps nearly as difficult to 

 determine as absolute resistance; and the error thence arising 

 doubles itself in the result. There is in addition the error 

 of K. But even if H and K were not subject to error at all, 

 I believe that the occurrence of the fourth power of the radius 

 of the inductor is a fatal defect, and tends to explain the dis- 

 crepant result obtained by Kohlrausch*. It is also worthy of 

 note that the error of levelling enters twice as much as in II., 

 III., and IV. 



VI. Lorentz's Method. 



This method, which, with the introduction of certain modi- 

 fications not affecting its essential character, I am disposed to 

 consider the best of all, was proposed and executed by Lorentz, 

 of Copenhagen, in 1873 f. A circular disk of metal, main- 

 tained in rotation about an axis passing through its centre at 

 a uniform and known rate, is placed in the magnetic field due 



* Oct. 1882. — It is very satisfactory to note that Kohlrausch (Gott. Ges. 

 Sept. 1882) has recently detected an error in the value of the area of the 

 windings of the inductor assumed in his previous calculations. Introdu- 

 cing the new value, obtained by an electrical process analogous to that 

 described in Maxwell's 'Electricity,' § 754, he finds 



1 B.A. unit = -390 xlO 9 . 

 t Pogg. Ann. cxlix. p. 251. 



