Read the given passage and answer the questions number that follow :
We know that accurate measurement of an unknown resistance can be performed on a Wheatstone bridge: However, for measuring the resistance of an ionic solution we face two problems. Firstly, passing direct current (DC) changes the composition of the solution. Secondly, a solution cannot be connected to the bridge like a metallic wire or other solid conductor. The first difficulty is resolved by using an alternating current (AC) source of power. The second problem is solved by using a specially designed vessel called conductivity cell. Basically it consists of two platinum electrodes coated with platinum black (finely divided metallic Pt is deposited on the electrodes electrochemically). These have area of cross-section equal to 'A' and are separated by distance 'l'. Therefore, solution confined between these electrodes is a column of length 'l' and area of cross-section 'A'. The resistance of such a column of solution is given by the equation :
`R = rho 1/A = 1/(kA)`
Name the instrument for accurate measurement of an unknown resistance.

Explanation

Wheatstone bridge.