352 M. F. Kohlrausch on the Absolute Value of the 



resistance 1 Siemens at + 12°0, the pieces in question accurately 

 represent tenths at that temperature. 



As the circuit inductor + galvanometer had been brought to 

 nearly 4 Siemens by the introduction of a small load of copper 

 wire, a differential galvanometer could be used. It was exter- 

 nally similar to that described in page 345 et seq. To be quite 

 free from thermal influences, the comparison was made by means 

 of short currents produced by Weber's magnet inductor by the 

 method of multiplication, and which were divided by the two resist- 

 ances and the two galvanometer wires. The heating by the very 

 feeble brief currents is in any case vanishingly small ; at the same 

 time accidental thermo-electromotive forces, which in a circuit 

 extending over a large space are unavoidable, have no influence, 

 as the induction-currents alternate in direction. The method 

 used, especially with reference to the extra current produced in 

 the inductor, has been described by me in Poggendortf '& Annalen, 

 vol. cxlii. p. 418 ; and I refer to that paper. 



The arrangement of the whole of the instruments is represented 

 schematically, fig. 2. C is a stopper commutator with six copper 

 plates made of ebonite, massive, and very carefully worked. Near 

 it are five solid clam ping- screws, which can be placed in conduct- 

 ing communication with each other by a slider. The connexion of 

 the other parts is shown by the figure. J and G, terrestrial induc- 

 tor and galvanometer, form the circuit the absolute resistance 

 of which is to be determined. These are closed at i by stoppers. 

 For comparison with Siemens' s standard, 1 is removed and 2 

 stopped. The standard E is then connected with one, and the 

 circuit J G with the other branch of the differential multiplier I). 

 To change them in reference to these branches, it is simply ne- 

 cessary to take out the stopper at 2 and insert it at 3. M is the 

 source of the current, the magnet inductor (which was removed 

 during the absolute measurements). The tenths added to E or 

 J G are denoted by 0*1 ; by sliding and fixing the shunt they 

 become ineffective. 



The needles of the galvanometer G were, of course, fixed du- 

 ring the comparison. It was specially ascertained that the 

 motion of the inducing magnet in M, which, as is well known, 

 consists of two magnets with similar poles presented to each 

 other, had no action at a distance on the inductor. 



It is useless to detail the experiments, since the errors of com- 

 parison cannot in any case compare with those of absolute deter- 

 mination. Moreover the example deduced in the memoir already 

 cited (p. 421) is oue of these determinations. 



Nos. la. and lb. of the absolute measurements belong to a 

 comparison made between them with the provisional standards, 

 as the Siemens's standards had not then reached me. II. and 



