THE BACK CURRENT 69 



the latter the movement of the wire WW can be 

 readily controlled, for if the line resistance be w\ 

 and the resistance at R be w 2 , then the ratio of the 

 current entering the " string " WW to that 



absorbed by the shunted resistance will be . In 



w\ 



considering displacements of the string which are 

 very small compared with its length, we may regard 

 the displacement as proportional to the current. 

 Hence, by varying w%, w\ remaining constant, the 

 displacement can either be regulated to work with 

 a slit of any desired width or to accommodate a 

 current received of any strength, the width of slit 

 remaining constant. 



In much of the work the battery E was put in 

 series with the resistance R, so that current flowed 

 continuously through the string and gave it a dis- 

 placement opposite in direction to that caused by 

 the line current. This procedure conduces to 

 " dead-beatness," and it brings the string back to 

 the zero position very rapidly the moment the line 

 current is interrupted. The ratio of the " reverse " 

 current to the " line " current is best varied to suit 

 the circumstances, but if the latter be fairly uniform, 

 the most desirable plan from the operator's point 

 of view is to keep E and R constant, and to adjust 

 the movement in the string caused by the line 

 current by means of a regulating resistance in series 

 with the line. 



