The interfacial tension of transformer oil is typically measured using a tensiometer, which quantifies the force required to break the surface between two immiscible liquids. One common method involves the Du Noüy ring method, where a platinum ring is submerged in the oil and then pulled upward until the ring breaks through the liquid surface. The force exerted at this point is converted into interfacial tension values. Another technique is the Wilhelmy plate method, which uses a thin plate immersed in the transformer oil and measures the force acting on the plate as it interacts with the oil’s surface. Both methods require careful calibration and standardization to obtain accurate interfacial tension readings, which are crucial for assessing the oil’s performance and compatibility in electrical applications.