1895.] on the Absolute Measurement of Electrical Besistance. 605 



velocity. That would be to neglect the unknown dimensions of the 

 magnetic permeability of the ether.] 



If the unit of length, the unit of time, and the properties of the 

 ether remain constant, this unit of electrical resistance remains con- 

 stant. It is unconditioned by the properties of any material, by 

 position in space, or point of time, and so far deserves the name of 

 the absolute unit of electrical resistance. 



How comes it that resistance can be so measured ? The answer 

 to this question is best found in a description of some one of the 

 methods by which the measurement of an electrical resistance in terms 

 of the absolute unit can be experimentally made. 



And I proceed, therefore, to a description of the method which 

 may fairly be described as the simplest, and which, in my opinion, 

 having regard to the magnificent possibilities of mechanical engineer- 

 ing operations in this country, is undoubtedly capable of being made 

 the most accurate of all the methods that have been proposed since 

 the British Association Committee, more than thirty years ago, pro- 

 pounded the theoretical definition of the absolute unit. 



The method is clue to Lorenz ; and Lord Eayleigh, at the conclu- 

 sion of his masterly determinations of the value of the B.A. coils in 

 absolute measure, expressed himself in regard to it as follows : — 



" On the whole, I am of opinion that if it is desirable at the 

 present time to construct apparatus on the most favourable scale so 

 as to reach the highest attainable accuracy, the modification of 

 Lorenz's method last described is the one that offers the best prospect 

 of success." 



The Paper from which I quote contained a comparison of the 

 various methods of measuring resistance in absolute units, and an 

 invitation to others to join in the work. It was the starting-point of 

 my own researches in the matter, which have extended over some 

 years, and I gladly take the opportunity of thanking Lord Eayleigh 

 for this source of inspiration. 



Faraday discovered that, if a conductor is made to move in a 

 magnetic field so as to cut across the flux of magnetic induction, the 

 conductor becomes the seat of electromotive-force, and that the electro- 

 motive-force so developed is proportional to the rate at which the 

 induction flux is traversed. 



If, for instance, we take a metal disc and make it rotate about a 

 horizontal axis n times a second, any radius of the disc cuts through 

 the earth's induction flux, unless the plane of the disc is in the mag- 

 netic meridian. There will, therefore, be electromotive-force between 

 the centre and circumference of the disc. Further, since a radius 

 traverses the whole area of the disc n times in a second, the rate at 

 which the induction flux is being traversed by any radius is n I, 

 where I is the total flux of induction through the disc area. But the 

 electromotive-force is proportional to this rate. Therefore we have 

 (with a proper choice of the unit of electromotive-force) 



E 2 = wl, 



