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