The Thiele modulus was developed by Ernest Thiele in his paper 'Relation between catalytic activity and size of particle' in 1939. Thiele reasoned that a large enough particle has a reaction rate so rapid that diffusion forces can only carry the product away from the surface of the catalyst particle. Therefore, only the surface of the catalyst would experience any reaction. The Thiele modulus is represented by different symbols in different texts, but is defined in Hill as hT.