THE first attempts at an explanation of magnetic phenomena were mainly concerned with the problem of action at a distance, but later the centre of interest shifted to the mechanism within the magnetized body which came into action under the influence of the external field. After the discovery by Oersted of the magnetic effect of an electric current, Ampère suggested in 1825 that a hypothesis of molecular currents might explain induced and permanent magnetization. At the time, only ferromagnetic phenomena were known, but the design of powerful electromagnets enabled Faraday to make the fundamental discovery that all matter is affected to a greater or less degree in a magnetic field. There are two broad types of behaviour: diamagnetic substances tend to move from the stronger to the weaker part of a non-uniform field and to set their longest dimension at right angles to the lines of force; paramagnetic substances move from the weaker to the stronger part of a field and set their length along the lines of force. Faraday showed that the resultant flux (lines of force per square centimetre) within a diamagnetic body must be less, while the flux within a paramagnetic body must be greater, than that in the external field. That means that the induced magnetization within a diamagnetic body opposes the inducing field, while that within a paramagnetic body reinforces the field.