A block that can detect temperature of nearby blocks, the thermometer will stack up temperatures from all adjacent blocks and display it. It will also emit a redstone signal when the temperature it detects surpasses a set amount. The amount can be altered by right clicking the block, using a wrench will change the temperature in the opposite direction. This is particularly useful in Fission Reactors to prevent overheating and meltdown. However, a manual switch is also recommended as backup.