It is on Joule's thombstone!

He did this mechanically, with a weight that dropped driving a "mill" in a calorimeter with water: https://commons.wikimedia.org/wiki/Category:Mechanical_equivalent_of_heat

Orignally by James Prescott Joule in 1843. It is on his thombstone!

Joule did this mechanically, with a weight that dropped driving a "mill" in a calorimeter with water: https://commons.wikimedia.org/wiki/Category:Mechanical_equivalent_of_heat