from from www.ghost-sighting.co.uk/
Thermometer / Thermal Scanner. Thermometers are an instrument that are also very useful. There are two types used regularly: digital thermometers and infrared non-contact thermometers. When used on an investigation this will aid as a detection system for spirit presence. Rapid temperature drops of 10 degrees or more could indicate spirit activity. I recommend using the infrared non-contact thermometers because they react in less than a second to temperature drops and can scan a large area quickly.
As mentioned, good thermometer can detect changes in room temperature quickly. But some still use the old-fashioned mercury-filled, red-line, thermometers. Electronic thermometers should be used, but in high paranormal, haunting activity where electromagnetic forces have been reported they may fail right when you need them. So bring an old mercury thermometer with you as back up, and you should be o.k. during your ghost investigation.