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Magnetic effect of current


A current carrying wire produce a magnetic field in its vicinity. This field can be detected by a small magnetic compass places close to it. Following demos are variants of Oersted experiment.

  1. Oersted experiment
  2. Circular coil
  3. Electromagnet

Oersted experiment

Let a wire is connected to a battery and a switch. When switched is made on, a magnetic compass placed nearby gets deflected. This was demonstrated for the first time by Oersted. A compass can acts as a current detector or galvanoscope.

You need a magnetic compass, battery, wire, and a small plastic box.

Place a magnetic compass on a plastic or wooden block, away from all magnetic material. When the compass needle comes to rest, fix a wire over the compass, parallel to the needle. Connect the wire to a battery through a switch, as shown in figure. Close the switch to pass a current through the wire. The compass needle will get deflected, and comes to rest at right angles to its original position. If the direction of the current is from south to north, the north pole of the needle will come to rest pointing west.


Now, hold the compass above the wire. The needle will get deflected in the opposite direction. The direction of deflection will also change if you reverse the direction of the current in the wire by interchanging the battery connections. If you switch off the current in the wire, the needle will go back to its original position.

Make sure that you pass a current through the wire only for short period of time (say, 5 seconds). Allowing current to pass through the wire for a long time will heat the wire considerable and also drain the battery rapidly.

Circular coil

If a magnet is kept near a current carrying wire, it tries to align itself in the direction of the magnetic field produced by the current. This fact is used to make a device which can detect current. In this demonstration we make one such current detector.

current detector

You need a coil made of about 30 turns of enameled copper wire fixed on a plastic stand using M-seal, a magnetized needle, connecting wires and a battery.

  1. Suspend the magnetized needle in the middle of the coil with a thread.
  2. See that, in equilibrium position, the needle points in the north-south direction. Why?
  3. Now connect the two ends of the coil to the battery.
  4. The needle swings and comes to rest in a direction perpendicular to the plane of the coil. Why?

When current passes through a coil it establishes a strong magnetic field which points in a direction perpendicular to the plane of the coil at the center of the coil. So the needle swings in the direction of the magnetic field.

Point of discussion: In the absence of the current in the coil, the needle is visibly affected by the earth's magnetic field but when current flows in the coil, the magnetic field of the coil is so strong that the effect of earth's magnetic field on the needle is not visible.

How to make an electromagnet?

An electromagnet is made by passing electricity through an insulated wire wound around a soft iron core. When an electric current is passed through a wire, the soft iron core becomes magnetized. It looses its magnetic properties as soon as the flow of electric current is switched off. The strength of an electromagnet depends upon the number of turns in the coil, the strength of current through the coil, and the properties of the core material.

Hazard: Never connect the coil/electromagnet to the 220 V supply. It may be fatal.


Wind a solenoid directly over an iron bolt or nail, using 5-8 feet of thick enamelled copper wire. (In these wires, the enamel coating acts as an insulator.) Use adhesive tape to keep the turns in place. Remove the enamel from the free ends of the wire, and connect them to a battery through a switch. Now, close the switch. The bolt will attract iron and steel objects placed near it. If you turn the current off, the bolt will no longer attract these objects.


Test the strength of the electromagnet by attaching pins one below the other. Wrap one or two extra layers of the coil over the first one and test the strength again. Bring the electromagnet near a compass to test its polarity. Change direction of current and again test its polarity.

Now connect the electromagnet to two or three batteries in series and see the strength. Remove the batteries and see what happens.

The soft iron core makes a stronger electromagnet as it can be magnetized easily. The polarity of electromagnet depends on the direction of flow of current.


  1. Experiments

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