In the post office box circuit, 10Ω  plug is taken out in arm AB and  100 Ω plug is taken out in arm BC. If the unknown resistor is kept in melting ice chamber, 600 Ω  resistance is required in arm AD for zero deflection in galvanometer. Now, if the unknown resistor is kept at 100°C  (steam chamber), 630 Ω  resistance is required in arm AD for zero deflection. Temperature coefficient of resistance of the unknown wire is found to be  p×10−4/C°. Find the value of p ?