Determination of Resistances in Absolute Measure. 343 



degree of accuracy aimed at, and ultimately adopted a method 

 of shunting. The main current from the battery was divided 

 into two parts, the larger of which passed through a resistance 

 of half a unit, formed by combining two singles in multiple 

 arcs. The resistance traversed by the other part of the main 

 current was much larger (from 10 to 20); and it was to two 

 points on this branch distant -^ that the wires of the derived 

 circuit were connected. With proper precautions this arrange- 

 ment was found satisfactory, and the equivalent resistance R 

 could be accurately expressed in terms of the standard B.A. 

 units. The adjustment for obtaining the balance was effected 

 by varying a large resistance placed in multiple arc with one 

 of the others ; or rather two effective resistances were used, 

 one on either side of that required for balance, the latter being 

 finally calculated by interpolation from the indications of the 

 galvanometer. 



By observing only the effect of reversing the battery-cur- 

 rent the results are freed from the influence of terrestrial 

 magnetism, and from the very sensible thermoelectric force 

 having its seat at the sliding contact. These contacts were 

 made by means of brushes of copper wire. One brush pressed 

 against the cylindrical edge of the disk, which was about 

 I inch broad; and the other pressed against the shaft on which 

 the whole turned. The area included by the secondary circuit 

 was therefore not exactly that of the disk, but required a small 

 correction, as to which, however, there is no difficulty. 



The arrangements for driving the disk and for observing 

 the speed were the same as for the revolving coil of method III. 

 The results, which in the same arrangement have not differed 

 by so much as yooo on different days, show that the sensitive- 

 ness was sufficient. 



After these explanations I come to the main subject of the 

 present remarks, viz. the degree of accuracy likely to be 

 attained in the fundamental linear measurement. In the pre- 

 sent case the quantity to be determined is M; and so far there is 

 no difference between this method and I. But the fact that 

 the secondary circuit is here represented'by a disk whose dia- 

 meter can be measured much more accurately than that of a 

 coil introduces a certain modification. It is necessary also that 

 the arrangements be symmetrical with respect to the middle 

 plane of the disk, as, on account of the width of the brush, the 

 place of contact cannot be considered as well defined. The 

 necessary condition can be satisfied with a single coil by placing 

 it so that its mean plane coincides with that of the disk. In 

 this position slight errors of adjustment produce effects of the 

 second order only, and every thing depends upon the radii. 



