# How do we know which direction heat flows? From cold to hot, or from hot to cold?

I'm trying to figure out which direction the heat flows. After many google searches I've determined it flows from hot (higher energy) to cold (lower energy).

My question tho is, how do we experimentally prove that it's indeed the case? Can it be "seen" flowing somehow?

• Note: energy flows via heat in the direction of higher temperature to lower temperature. This is not the same thing as higher energy to lower energy. – march Nov 12 '15 at 21:37

Once you have a thermometer you can measure the temperatures of two different objects, say $T_1$ and $T_2$. It is an experimental fact that, after putting in contact two such objects with $T_1 > T_2$, the system will reach an equilibrium state with the two bodies having the same temperature (that, again, can be measured via a thermometer).