The mechanical action on iron of the first horseshoe electro-magnets (1824) was obvious. Their use to investigate more subtle magnetic, magneto-optical, atomic or nuclear properties of matter began in 1845 with Faraday. Until the first iron-less Bitter magnets in 1933, and 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 evolution of iron-cored laboratory electromagnets from Faraday to Ruhmkorff, then to du Bois and Pierre Weiss, culminating in the giant electromagnets built between WWI and WWII. The electromagnet published by Weiss in 1907 became the archetype of many later ones. The achievable field limitation due to iron saturation was overcome with iron-less solenoids, first with resistive windings (Bitter), later with superconductive coils. A series of fundamental physics discoveries, were made, often made using rather primitive electromagnets.