The resemblances of electricity and magnetism did not escape attention, and the derangement of the compass needle by the lightning flash, formerly so disastrous at sea, pointed to an intimate connection between them, which was ultimately disclosed by Professor Oersted, of Copenhagen, in the year 1820.  Oersted was on the outlook for the required clue, and a happy chance is said to have rewarded him.  His experiment is shown in figure 29, where a wire conveying a current of electricity flowing in the direction of the arrow is held over a pivoted magnetic needle so that the current flows from south to north.  The needle will tend to set itself at right angles to the wire, its north or north-seeking pole moving towards the west.  If the direction of the current is reversed, the needle is deflected in the opposite direction, its north pole moving towards the east.  Further, if the wire is held below the needle, in the first place, the north pole will turn towards the east, and if the current be reversed it will move towards the west.

The direction of a current can thus be told with the aid of a compass needle.  When the wire is wound many times round the needle on a bobbin, the whole forms what is called a galvanoscope, as shown in figure 30, where N is the needle and B the bobbin.  When a proper scale is added to the needle by which its deflections can be accurately read, the instrument becomes a current measurer or galvanometer, for within certain limits the deflection of the needle is proportional to the strength of the current in the wire.

A rule commonly given for remembering the movement of the needle is as follows:—­Imagine yourself laid along the wire so that the current flows from your feet to your head; then if you face the needle you will see its north pole go to the left and its south pole to the right.  I find it simpler to recollect that if the current flows from your head to your feet a north pole will move round you from left to right in front.  Or, again, if a current flows from north to south, a north pole will move round it like the sun round the earth.

The influence of the current on the needle implies a magnetic action, and if we dust iron filings around the wire we shall find they cling to it in concentric layers, showing that circular lines of magnetic force enclose it like the water waves caused by a stone dropped into a pond.  Figure 31 represents the section of a wire carrying a current, with the iron filings arranged in circles round it.  Since a magnetic pole tends to move in the direction of the lines of force, we now see why a north or south pole tends to move round a current, and why a compass needle tries to set itself at right angles to a current, as in the original experiment of Oersted.  The needle, having two opposite poles, is pulled in opposite directions by the lines, and being pivoted, sets itself tangentically to them.  Were it free and flexible, it would curve itself along one of the lines.  Did it consist of a single pole, it would revolve round the wire.