The mechanical action on iron of the first horseshoe electro-magnets (1824) was obvious. They quickly found important applications (telegraphy). Their use to investigate more subtle magnetic, magneto-optical, atomic or nuclear properties of matter began in 1845 with Faraday. Until the 1970s, when superconducting magnets became common, iron-cored electromagnets were normally used to produce steady magnetic fields of high intensity. We will follow the history of a series of fundamental physics discoveries, often made using rather primitive electromagnets, as well as the evolution of laboratory electromagnets from Faraday to Ruhmkorff to Pierre Weiss. The electromagnet published by Weiss in 1907 became the archetype of many later ones.