A Maximum Minimum Temperature System or MMTS is a temperature recording system that keeps track of the maximum and minimum temperatures that have occurred over some given time period.

The earliest, and still perhaps most familiar, form is the Maximum minimum thermometer invented by James Six

James Six

James Six was a British scientist born in Canterbury. He is noted for his invention, in 1780, of Six's thermometer, commonly known as the Maximum minimum thermometer...

 in 1782.

Today a typical MMTS is a thermistor

Thermistor

A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...

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This may be read locally or can transmit its results electronically.