The temperature of the winding is measured and determined indirectly. The temperature difference between the winding and the coolant is a function of the actual current in the winding. The transformer secondary current is proportional to the current in the transformer winding. The secondary current of the transformer feeds a heating resistor in the thermometer and so produces a display reading of the transformer load, corresponding to the actual measured oil temperature (temperature gradient). This indirect method provides a display of the average or maximum winding temperature (i.e.a thermal image). The heater current is set to a characteristic curve using a potentiometer located in the dial indicating thermometer. It can be set in the factory or by the customer.