The Thermometer is one of the gadget that you will use to detect the presence of a ghost in the area. Leave the Thermometer on the wall on lighthouse and even if you are away you would know when there is a ghost where you place the Thermometer.
How to Get the Thermometer
Head out of the house and then run over to Main Street. Go inside the bakery and talk to Jane the baker. Tell her, “Fiona sent me.” She then walks over to the pastry case and it sinks into the ground. A new case rises up to replace it and it contains all kinds of cool ghost hunting gear! One of which is the Thermometer.