The phenomenon Faraday identified in 1845, showing that matter responds to magnetic fields, is diamagnetism.
Diamagnetism is a weak magnetic response in which an induced magnetic field opposes an applied magnetic field, causing repulsion. Faraday investigated how different materials behaved between the poles of a powerful electromagnet and concluded that all matter responds magnetically, either diamagnetically or paramagnetically.
Faraday’s work gave the phenomenon its name, with “dia-” suggesting across or through. The discovery was part of his broader effort to understand magnetic lines of force and their relationship to matter. Earlier observations of magnetic repulsion existed—Anton Brugmans had reported bismuth’s repulsion in 1778—but Faraday established the response as a general property of materials.
Diamagnetism is usually weak because it arises from induced changes in electron motion. Materials with unpaired electrons may instead show stronger paramagnetism or ferromagnetism, which can mask their diamagnetic response. Superconductors are an especially dramatic example of diamagnetic behavior because of the Meissner effect.