Determination of Resistances in Absolute Measure. 333 



same time the subjects of the linear measurements would be 

 of more favourable magnitudes. To enhance the latter advan- 

 tage, it would probably be an improvement to diminish the 

 radial breadth of the section, on which much of the uncertainty 

 of mean radius depends. In either case it is clear that the 

 limit of accuracy obtainable by this method has not yet been 

 reached. 



II. Weber's Method by Transient Currents, Maxwell § 760. 

 "A coil of considerable size is mounted on an axle so as to 

 be capable of revolving about a vertical diameter. The wire 

 of this coil is connected with that of a tangent-galvanometer 

 so as to form a single circuit. Let the resistance of this 

 circuit be R. Let the large coil be placed with its positive 

 face perpendicular to the magnetic meridian, and let it be 

 quickly turned round half a revolution. There will be an 

 induced current due to the earth's magnetic force; and the 

 total quantity of electricity in this current in electro-magnetic 

 measure will be 



Q= 2 $A 



H R * 



where g x is the magnetic moment of the coil for unit current,, 

 which in the case of a large coil may be determined directly 

 by measuring the dimensions- of the coil and calculating the 

 sum of ' the areas of its windings; His the horizontal compo- 

 nent of terrestrial magnetism; and R is the resistance of the 

 circuit formed by the coil and galvanometer together. This 

 current sets the magnet of the galvanometer in motion." 



" If the magnet is. originally at rest, and if the motion of 

 the coil occupies but a small fraction of the time of a vibra-, 

 tion of the magnet, then, if we neglect the resistance to the . 

 motion of the magnet, we have, by § 7,48, , 



Q=^2sin-|0, 



where Gr is the constant of the galvanometer, T is the time of' 

 vibration of the magnet, and 6 is the observed elongation. 

 From these equations we obtain 



R =^fe 



The value of H does not appear in this result, provided it is 

 the same at the position of the coil and at that of the galva- 

 nometer. This; should not be assumed to be the case, but 

 should be tested by comparing the time of vibration of the- 



