In the early years, automobiles had their radiator caps outside of the hood and on top of the grille which also served as an indicator of the temperature of the engine's coolant fluid. The Boyce MotoMeter Company was issued a patent in 1912 for a radiator cap that incorporated a thermometer that was visible to the driver with a sensor that measured the heat of the water vapor, rather than the water itself. This became a useful gauge for the driver because many early engines did not have water pumps, but a circulation system based on the "thermo-syphon" principle as in the Ford Model T.