Resistance according to an absolute Standard. 237 



The current produced in the closed conductor by the electro- 

 motive force arising from the bar magnetism of a magnetic 

 needle oscillating in the centre of the ring, itself reacts accord- 

 ing to the principle of deadening on the oscillating needle, and 

 produces a diminution in the amplitude of its oscillations which 

 can be observed with great accuracy ; and the intensity of this 

 current may also, from these observations, be determined accord- 

 ing to an absolute standard with great accuracy. It is then 

 evident that the current does not need to be passed through a 

 second ring serving as galvanometer, in order to measure the 

 intensity of the current. Hence the whole conductor, whose 

 resistance is to be measured, can be used to form a single ring 

 which serves at once for indicator and multiplicator. 



According to this simplification, the observation of the arcs of 

 oscillation of a magnetic needle oscillating in the centre of the ring 

 is sufficient : by their magnitude the strength of the electromo- 

 tive force, and by their decrease the intensity of the current 

 produced in the closed conductor by that electromotive force, 

 can be determined. 



In executing the observations according to this principle of 

 deadening, it is of prime importance that the magnetism of the 

 needle oscillating in the centre of the ring be very powerful ; 

 and also that the length of the needle be very small as com- 

 pared with the diameter of the ring, in order that, in calculating 

 the resistance, there shall be no necessity for an accurate know- 

 ledge of the distribution of the magnetism in the needle, the 

 determination of which would be difficult. In the ring now 

 solely used, which is that previously called B, and which has 

 30351 millims. internal, and 374*41 millims. external diameter, 

 and is 202 millims. in height, a magnetic needle 90 millims. long, 

 and as strong as possible, was suspended. The experiment was 

 commenced by detaching from each other the ends of the wire 

 forming the ring. The needle was then set in oscillation, and 

 its time of oscillation and the decrease of its amplitude, or the 

 logarithmic decrement of this decrease, was determined accord- 

 ing to the method given by Gauss in the ' Results of the Obser- 

 vations of the Magnetic Verein in the year 1837*/ Thereupon 

 the annular conductor was closed, and the same observations 

 repeated. The results of these observations are given in the fol- 

 lowing Table, in which the logarithmic decrement of the diminu- 

 tion of the arc of oscillation with a closed conductor, stands in 

 the first column under A, the same with an open conductor 

 stands under B, while in the third column under t is given the 

 observed time of oscillation. The mean values are indicated 

 underneath : — 



* See Taylor's Scientific Memoirs, Part VI. Vol. II. 



