Properties y Specific Resistance, and Hardness of Steel. 355 



The following Tables IV. and V. contain the specific resist- 

 ances of the rods already cited in Tables II. and III. The 

 data are given in terms of mercury. The column S contains 

 the total specific resistances, AS the corresponding excess of 



AS 

 the latter over that of the normal rod I. Of the ratio T p °^ 



mention will be made hereafter. * ' ' 



Let us denote by a and b the resistances of the two parts of 

 the first branch of the bridge on each side of the sliding con- 

 tact, by K' and K" the resistances of the corresponding parts 

 (thick copper wire) of the second branch, into which the un- 

 known resistances W and R are inserted ; finally, by 8 the 

 standard of comparison above referred to, in terms of which 

 W and R are to be determined. When the current in the 

 galvanometer is zero, we shall have for a particular position 

 of the commutator, if we introduce 



(1) W and R alone 



b"n+K //S 



(2) W and R with 8 on the right, 



a f W + K7 



V R + K ;/ + 8 5 



(3) W and R with 8 on the left, 



W R + K" 



Three similar equations may also be derived for the other 

 position of the commutator, as only W and R are interchanged. 

 From these six equations we deduce: — 



First position, 



R K7' 



8 + 8 



I" 



Second position, 



W K7 ' _ 1 



8 8 a' a 



R K'_ 1_ 



8 + 8 ~ b" h 



