The thermometer is used to measure the temperature of transformer oil. These products are generally installed on the transformer to measure the high temperature of the oil. The high oil temperature is measured directly through a sensor installed on the tank cap.
The coil hot spot is assumed to be near the top of the coil where it is surrounded by oil. Therefore, this product measures the oil temperature, but the temperature difference between the coil and the surrounding oil must also be added to it.
The coil thermometer actually consists of an oil temperature measuring sensor and a thermal resistance (heater) wrapped around it. This heater is fed through a current transformer, with the sample current passing through the transformer, which leads to an increase in the temperature around the sensor.