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).