Many of these thermostats are the mercury bulb type which uses mercury to make contacts to energize the heat or cooling. Mercury is a highly toxic liquid conductive metal and needs to be handled with care. Human exposure to mercury is hazardous and if the bulb containing the mercury is broken great care should be taken to clean it up and dispose of it properly. Additionally, when the thermostat is replaced the old mercury type should be disposed of at an authorized disposal agency. Some mechanical thermostats do not contain mercury but use a calibrated magnetic strip and a bi-metal spring. These thermostats are effective but not as popular as the mercury type. Most of the non-programmable mechanical thermostats can be calibrated with special tools. It is recommended that only a qualified HVAC Technician perform these calibrations. The non-programmable mechanical thermostat comes in many different varieties for many different applications.