Hot water bag quality ranking

The liquid in the hot water bottle contains salt and iron. Salt is the kind that uses two electrodes to conduct electricity and the liquid as a conductor to heat, and iron is the kind that uses the magnetic effect to heat. If you use iron or stainless steel as a heater, and placed in the liquid has nothing to do with the same principle as the hot water bag chart. My magnet to judge my hot water bottle found that the middle cylinder is not magnetic, there are two small columns with magnetic at both ends, so I judge my heating rod is used to conduct electricity, the liquid is used as a heater, just like connecting electricity to a resistance wire. Only when the liquid conducts electricity can it heat up.

There are two possibilities. One is a bimetal. The use of different metal thermal expansion coefficient differences, when the temperature rises to a certain set value, induction hot water bags  ten bimetal contacts will jump open, thereby cutting off the heating power supply, automatically stop heating. The other is a thermistor, which can produce a sudden change in resistance at the set value, installed in the control circuit of the electric water heater. As soon as the hot water cylinder reaches the set temperature, the resistance value of the resistor becomes large, preventing the current from passing through and achieving the purpose of cutting off the power. (If smaller, there are thyristors, triggered by the thermistor to cut off power.