It has been known for a thousand years or more (originating in China) that certain (magnetic) materials would always orientate themselves in a particular direction if suspended to rotate freely. The very earliest experiments in magnetism were done with these materials (permanent magnets) and these clearly showed that two pieces of these materials were able to exert some force at a distance. This force is analogous to gravitational force. We know from our own experience that it exists. Physicists theorise on the causes whilst engineers are more concerned with being able to measure the practical effect and put it to use by devising a suitable method of analysis.Very early experiments by Oersted and Ampere showed that a current carrying conductor also had an effect on magnetic material in its vicinity. Magnetic compasses placed near to a current carrying conductor were deflected. He also found that the direction of deflection depended on the position relative to the wire. Those above the wire were deflected in the opposite direction to those placed below. Ampere quantified the strength of this force in terms of the current and the distance involved. In order to be able to relate these observations to analysis, the concept of a magnetic field was introduced.The presence of a magnetic field may be visualised by drawing imaginary continuous lines of 'magnetic flux', the density of which is a measure of the strength of the field in a given material. Arrows are added to the flux lines to indicate the direction of the magnetic field, from which the direction of the force it produces on, for example, compass needles and current carrying conductors can be deduced. Convention has it that the magnetic field strength is denoted by the symbol H (ampere.turns), whilst magnetic flux density is given the symbol B (Webers/m2).