Ox THE ABSOLUTE UNIT OF ELECTRICAL RESISTANCE 147 



meter by means of a shunt, while all the induced current passed through 

 it. But, owing to the heating of the wires, the shunt ratio cannot be 

 relied upon as constant, and hence the defect of the method. At pres- 

 ent this experiment has only historical value, seeing that no exact 

 record was kept of it in a standard resistance. However, we know that 

 the wire was of copper and the temperature R. and that the result 

 obtained gave the resistance of the wire $ smaller than Weber found 

 for the same wire at 20 R. in 1851. 



In 1851, "Weber published 8 experiments by two methods, first by 

 means of an earth inductor, and second by observing the damping of a 

 swinging needle. Three experiments gave for the resistance of the 



circuit 1903 -10 8 , 1898 -10 8 , and 1900 -10 s , , but it is to be noted 



sec. 



that a correction of five-eighths per cent was made on account of the 

 time, two seconds, which it took to turn the earth-inductor, and that 

 no account was taken of the temperature, although the material was 



copper. He finds for the value of the Jacobi unit, 598 -10 7 ^. Three 



OCC'B 



years after that, in 1853, Weber made another determination of the 

 specific resistance of copper. 4 But these determinations were more to 

 develope the method than for exact measurement, and it was not until 

 1862 5 that Weber made an exact determination which he expected to 

 be standard. In this last determination he used a method compounded 

 of his first two methods by which the constant of the galvanometer was 

 eliminated, and the same method has since been used by Kohlrausch 

 in his experiments of 1870. The results of these experiments were 

 embodied in a determination of the value of the Siemens unit and of 

 a standard which was sent by Sir Wm. Thomson. As the old Siemens 

 units seem to vary among themselves one or two per cent, and as the 

 result from Thomson's coil differs more than one per cent from that 

 which would be obtained with any known value of the Siemens unit, 

 we cannot be said to know the exact result of these experiments at the 

 present time. Beside which, it was not until the experiments of Dr. 

 Matthiessen on the electric permanence of metals and alloys, that a 

 suitable material could be selected for the standard resistance. 



The matter was in this state when a committee was appointed by the 



3 Elektrodynamische Maasbestimmungen ; or Pogg. Ann., Bd. 82, S. 337. 

 4 Abh. d. Kon. Ges. d. Wissenchaften zu Gottingen, Bd. 5. 



5 Zur Galvanometrie, Gottingen, 1862. Also Abb. d. K. Ges. d. Wis. zu Gottingen, 

 Bd. 10. 



