THE MEASUREMENT OF RESISTANCE 



181 



the points. This curve shows what the length of the section 

 would be if its lower end were at any point on the slide wire. 



Call the ordinate at x = 0, y\. If the lower end of the section 

 were at the upper end would be at x = y\. Look up the 

 ordinate at that point, call it 3/2, add it to y\ and determine y z 

 and so on. Let yi + i/ 2 . . . + y n = S. 



The resistances of the section S, denoted by R a , and of the 

 whole wire between and 100, denoted by Rwo, are then measured 

 by any convenient method. Obviously, the section S is made 



D 



up of n smaller sections each having M resistance '- 



Rw 



FIG. 103. Pertaining to calibration of slide wire. 



The resistance of the section 



o r> 



r 



is, of section 



+ ?/ 2 , 



of section y l + y z + 7/3, ~^', of section y l + y 2 + 2/s + . . + 2/, 



These resistances may be plotted as ordinates, using as 



abscissae the values, y i} y l + y z , y l + y 2 + 2/3, etc. (Fig. 103). 

 At the 100 mark #100 is laid off; a straight line drawn through 

 this point and the scale reading is the line along which the 

 points previously plotted would lie if the wire were uniform. 



Take any scale reading, /; the resistance of the length I is 

 RI. The corresponding reading on the uniform wire is obtained 



