The thermoscope — or differential thermometer — was invented in the very early nineteenth century by the physicial and chemist Benjamin Thompson, Count Rumford. It was an ingenious instrument for measuring the transmission of heat. The U-shaped glass tube containing a drop of liquid is attached to a wooden board, graduated on its horizontal side. When the temperature in one bulb exceeds that of the other, the air in it expands, causing the drop of liquid to move inside the tube.