When cell of E.M.F. ' $E_1$ ' is connected to potentiometer wire, the balancing length is $l_1$. Another cell of E.M.F. ' $E_2$ ' $\left(E_1>E_2\right)$ is connected so that two cells oppose each other, then the balancing length is $l_2$. The ratio $\mathrm{E}_1: \mathrm{E}_2$ is

The logic gate combination circuit shown in the figure performs the logic function of

In an isobaric process

A concave mirror of focal length ' $f$ ' produces an image ' $n$ ' time the size of the object. If the image is real, then the distance of the object from the mirror is