A love thermometer can refer to a number of very different products, including novelty love-themed hand boilers and simple programs that claim to give an objective arbitrary value to relationship success rates. The latter is also sometimes called a love calculator and is typically an application or website rather than a physical device.
A hand boiler is a device that uses the physics of the interaction of temperature and pressure inside of an enclosed space to create what appears to be liquid that floats up and begins to boil merely because a person places his hand on the bottom. In reality, the chemical inside of the hand boiler has a very low temperature point for evaporation. Simply placing a warm hand on the bottom of the device causes the liquid to begin evaporating; however, since the liquid is in an enclosed environment at a fixed pressure inside of the tube, it quickly condensates at the top part of the hand boiler and begins to bubble as long as the heat source remains in place.
Love calculators that appear to measure love are inconsistent between applications and have no hard science behind them. They typically take both party's names as an input and use them to generate a random number so that it gives the same results for the same names if used again.