The accidental discovery of a magnetic field around a current-carrying wire left the Danish Physicist, Oersted startled. It so happened one day that while he was experimenting in front of his students to show the process of the wire getting heated up when an electric current flows through it, he was spellbound to notice the magnetic needle of the compass located next to the wire rotate.

Oersted Experiment

  • The compass laid on a table needs to face upward and wait until it points toward the north. 
  • The middle of the wire needs to be laid above the compass needle in the north-south direction. In order to keep it in position the wire needs to be lightly taped to the table. 
  • One end of the wire needs to be connected to each end of the battery.
  • The wire needs to be immediately disconnected from the battery as it is bad for the battery to draw such a large current.
  • It needs to be repeated with the reverse connections of the battery every time.
  • Taking a piece of paper, fold the longer side into pleats. The wire is put on the table with its middle in the north-south direction. The pleated paper is placed above it so that the wire remains underneath the pleats and the compass is placed on top of the pleats.
  • The whole experiment can be repeated with the compass above the wire.

The wire carries a current creating a magnetic field around itself. When the compass is brought near the wire or in the loop, it causes the compass needle to move leading to the fact that the wire itself acts as a magnet.

The reason behind such happenings is the current inducing a magnetic field based on the right-hand rule. Making a thumbs-up sign with the right hand, the thumb will be in the direction of the current, that is, flowing from the negative terminal of the battery to the positive one and the fingers curve around in the direction of the magnetic field. The current creates the magnetic field which interferes with the magnetic field that the compass goes through when it is brought close enough.

Hans Christian Oersted

Being a worker for his own father who was the owner of a local pharmacy, Hans Christian Oersted (1777-1851) was highly interested in science. In 1820 he discovered the direct relationship between electricity and magnetism through the deflection of a compass needle from magnetic north by the flow of an electric current.

Oersted’s Law

Oersted discovered the following when a straight wire carries a steady direct current (DC)-

  • The magnetic field lines around the current-carrying wire.
  • The magnetic field lines on a plane at right angles to the wire.
  • The direction of the magnetic field alters when the direction of the current is reversed.
  • The magnitude of the current and the strength of the field are directly proportional to each other.
  • The distance of the point from the wire and the strength of the field at any point are inversely proportional.

Oersted’s law is limited to the steady currents which don’t change with time. So, it is applicable only for DC electric circuits without any capacitors or inductors. It is noticed that it fails for time varying currents.

This verification can be done through experiments in which the current in the circuit creates some magnetic field but any closed curve within circles the conductor can be spanned by a surface which passes between the capacitor plates through which no current flows. This leads to the equation giving zero magnetic field.

Oersted’s Observations

  • As the electric current passes through the wire from South to north direction and the wire is positioned over the compass needle, the compass gets deflected towards the west direction.
  • The compass needle gets deflected in the opposite direction when the direction of the current in the wire is reversed.
  • There is no deflection in the compass needle when the current is switched off.

Oersted thought of an electric current struggling through a wire. Because it flowed, this conflict gave rise to heat and light which seemed to radiate away from the wire. Connecting a voltaic pile to a platinum wire, he showed that it became hot and glowed.

Oersted Unit

The unit of the Oersted is Oersted itself given in the honour of the Danish Physicist, Hans Christian Oersted who discovered the connection between magnetism and electric current. The symbol Oe is the coherent derived unit of the helping magnetic field H in the centimetre-gram-second System of units or CGS which is equivalent to one dyne per maxwell.

In the SI system, the unit ampere per metre (A/m) is equivalent to Newton per weber used for the magnetic field.

Conclusion

As the electric current passes through the wire from South to north direction and the wire is positioned over the compass needle, the compass gets deflected towards the west direction.

The compass needle gets deflected in the opposite direction when the direction of the current in the wire is reversed.

There is no deflection in the compass needle when the current is switched off.