With a potentiometer null point were obtained at $140\, cm$ and $180\, cm$ with cells of $emf$ $1.1 \,V$ and one unknown $X\, volts$. Unknown $emf$ is .............. $V$
  • A$1.1$
  • B$1.8$
  • C$2.4$
  • D$1.41$
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