In an experiment to find emf of a cell using potentiometer, the length of null point for a cell of emf \( 1.5\,\text{V} \) is found to be \( 60\,\text{cm} \). If this cell is replaced by another cell of emf \( E \), the length of null point increases by \( 40\,\text{cm} \). The value of \( E \) is \( \frac{x}{10}\,\text{V} \). The value of \( x \) is ________.