Passage : Post office is useful to measure the value of unknown resistance correctly upto 2nd decimal place. It is based on Wheatstone bridge. Systematic diagrams of post office box are shown in the above figures. Each of the arms AB and BC contains three resistance of 10 Omega, 100 Omega and 1000 Omega. These arms are generally known as ratio arms. With the help of these resistance, we can introduce resistance P in arm AB and Resistance Q in arm BC. The resistance arm AD is complete resistance box containing resistance from 1 Omega "to" 5000 Omega. In this arm, we can introduced resistance R by taking out plugs of suitable values. The unknown resistance X is connected in fourth arm CD. These four arms actually form Wheatstone bridge shown in figure. For balanced conditions, no deflection is formed in galvanometer. At balanced condition PX= QR , X= (QR)/( P) While measuring surface tension of water using capillary rise method the necessary precaution to be taken is/are