Which physical constant did Cavendish effectively measure by weighing the Earth in a shed?

The gravitational constant - though Cavendish said he was measuring the density of the Earth, and never wrote G at all; the constant was extracted from his numbers by others a century later.

The apparatus was two small lead balls on a rod hung from a wire, with two large lead balls brought close. The faint gravitational attraction twisted the wire, and the twist was read off by telescope from outside the room, because a person in the room would have caused air currents that swamped the effect. The whole thing sat in a sealed shed in his garden. He measured a deflection of a fraction of a millimetre and got the Earth's density to within about one per cent of the modern value.

G remains the worst-measured fundamental constant, known to only about four or five significant figures while others are known to eleven, because gravity is so weak that nothing can be shielded from everything else and different laboratories keep getting incompatible answers. Cavendish himself was pathologically shy, communicated with his housekeeper by note, and left a mass of unpublished work - including electrical results that anticipated Ohm and Coulomb by decades and were found in his papers long after his death.