to the Wheatstone Bridge* 



419 



it substituted. The last three wires were then again tested, 

 and a marked improvement was at once seen, for now an 

 alteration of 1 ohm in x produced a difference of deflexion of 

 10-15 scale-divisions, thus giving the interpolated figure 

 correct to the first place given, i. e., R correct to 1 part in 3000. 

 To further test the capabilities of this method, a strand 

 cable composed of 7 copper wires, each of No. 16 S.W.Gr., 

 was taken, the strands being flattened to pass under the 

 springs, which had not been constructed for a wire of such a 

 large diameter. The results, using the same cell and resist- 

 ance, are shown in the next table. 



Table II. 



X. 



Observation. 



E. 



117 



6 divs. deflexion left. 





118 



H » „ left. 



0-001183 



119 



3 „ „ right. 





Here only 4J divisions correspond with a change in x of 

 1 ohm : thus an error in reading of J a division means an 

 error in R of 0*1 per cent. The effect of using a still larger 

 current was next tried, the 2-ohm coil being removed, and the 

 two resistances /?, p' of fig. 3 substituted. An ammeter was 

 also included in the battery-circuit, so that the actual current 

 could also be measured. 



Now, on passing a strong current both wires heat, and if 

 the wire r has the smaller diameter it will heat the faster. 

 Therefore its resistance will also increase the faster, and hence 

 the resistance of the wire R will apparently diminish, and 

 vice versa. In order to see how much this diminution might 

 amount to, the resistance p 1 was short-circuited, and p was 

 adjusted until the battery-current was 6 amperes, and the 

 results are shown in Tables III. and IV. 



Table III. 



With the current only continued long enough to measure 



the deflexion : current 6 amperes. 



X. 



Observation. 



E. 



118 

 119 



3 divs. deflexion right. 

 23 „ „ left. 



0-0011812 



2G2 



