Teaching Minds-On Experiments on Electromagnetism in Secondary Schools Oersted’s experiment – vertical version (or Ampere’s experiment)
We
place four small magnetic needles around a vertical straight wire (or
better a few wires in parallel), which is part of an electrical
circuit. When the switch is in the open position, we see that the all
compasses needles are in the north direction. (If they are too close
they tend to couple one with another and to form a kind or ring even
without the current in the wire).
When we close the switch and the wire carrying an electric current, the
compass needles turns with respect to circles around the wire. In this
experiment we show that electric current could be the source of a
magnetic force, and that the lines form circles.
The Ampere’s part of the experiment is that the intensity of the field
falls down as the inverse of the distance from the wire, and is
proportional to the current (1)
This follows from a general formulation of Ampere’s law applied for a
circular symmetry. As the direction of the current matches as well, if
current in two wires are opposite, the field is null.
