He accomplished this by using the ear canal's ear drum's blood vessels, which are shared with the hypothalamus. Before the ear thermometer's invention, easy temperature readings could only be taken from the mouth, rectum, or underarm.
He used the ear canal's eardrum's blood vessels, which are connected to the hypothalamus, to measure temperature.  Before the invention of the ear thermometer, other methods like the mouth, rectum, or armpit were used for taking temperatures.