A bimetallic strip is formed out of two identical strips, One of copper and the other of brass. The coefficients of linear expansion of the two metals are $\alpha _C$ and $\alpha_{B}.$ On heating, the temperature of the strips increases by $\Delta T$ and the strip bend to from an arc of radius R. Then R is proportional to :
$\begin {array} {1 1} (1)\;\Delta \;T & \quad (2)\;\frac{1}{\Delta T} \\ (3)\;\sqrt {\Delta T} & \quad (4)\;\frac{1}{\sqrt {\Delta T}}\end {array}$
$(2)\;\frac{1}{\Delta T}$