●●● ●●●● 2. Experimental principles ●●●●● ●●●● ●●●●● ●●●● 20 △P= R 20 20 R>I R=工 R>I Therefore, at the same temperature, once the aPl, AP2 are measured, and the IFT of the liquid(water) whose IfT is known is found out according to the temperature table, then we can calculate the ift of the unknown liquid from the formula。1 1 2 p r   = 2 2 2 p r   = 2 2 1 1 p p    =   Therefore, at the same temperature, once the ΔP1, ΔP2 are measured, and the IFT of the liquid(water) whose IFT is known is found out according to the temperature table,then we can calculate the IFT of the unknown liquid from the formula 。 （1） R P 2  = 2. Experimental principles