422 Determination of Low Resistances. 



for one direction of the current, a reversal produced no appre* 

 ciable deflexion, thus showing that this piece of wood had en- 

 tirely prevented such effects. As a precaution, however, the 

 current was reversed each time balance was obtained, and in 

 no instance was any deflexion caused by the reversal. 



The experiments were then conducted in the same order as 

 before. With the gap open, balance was obtained by moving 

 the slider I. The gap was then closed and balance obtained 

 by altering the position of the arm T C. The gap was next 

 opened, and the slider moved till again no deflexion was noted. 

 This motion was very slight, and the alteration, after closing 

 the gap, was not found to have produced any observable want 

 of balance. The correct position of C was thus determined. 



In order to see with what degree of accuracy this position 

 had been arrived at, the arm was shifted 1 millim. in both 

 directions, and the want of balance was indicated by a de- 

 flexion on either side of zero of one division ; hence, as 

 the length between B and C was over one metre, the error 

 in the position of C was well under 0*1 per cent. 



The arm TO was then soldered to the wire, and the tests 



repeated. No deflexion was observable with the ratio -^r«, 

 but on adding a 1-ohm coil to the 1000, making the ratio 

 -r^-r, the want of balance produced nearly 2 divisions de- 

 flexion. The wire was then screwed in its proper place and 

 there tested in a similar fashion with the same result, viz., 



with the ratio tttftr no deflexion was noted, while with the 



ratio TKryi a deflexion of over 1 division showed the want of 



balance. A thermometer lying beside the wire indicated 

 17°-7 C. Thus the resistance between B and C was O'Ol ohm 

 at 17°* 7 C, with an error of less than 0*1 per cent. 



Thus on all three conditions an accuracy of 0'1 per cent, 

 was obtained. This simple apparatus is, therefore, capable of 

 measuring the resistances of metre-lengths of wires between 

 the limits of No. 22 S.W.G. and a stranded cable of 7 

 No. 16's (and probably over a still greater range) with an 

 accuracy throughout of 0*1 per cent., an accuracy quite suffi- 

 cient for all commercial purposes. 



Although in the above simple form of The Addition a slide- 

 wire bridge is necessary, it may happen that this latter is not 



