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posted Jan 29, 2015, 2:39 AM by   [ updated Sep 19, 2016, 7:51 PM by Upali Salpadoru ]

Fig.1. A powerful electro magnet to pick up and drop magnetic materials.

Fig. 2. Magnetic lines arond a current carrying wire.

What is the direction? Clock-wise or counter clock-wise?   


This can be determined by using a small pocket compass. There is a rule to find this.

The Cork screw rule

There is a simple rule to remember the direction of the concentric magnetic lines of force induced by a current.


If you send a screw driver in the direction of the current, the way you have to turn it gives the direction of the force.


 Fig.3 Cork-screw rule.

Fig.4  Getting the direction of the force

Several loops of wire are wound using the holes made in a thick piece of paper.  The direction of the current is shown by the black arrows. When the current is going into the paper the magnetic lines are clockwise. Shown in red. When the current is coming up they are anti-clockwise. Shown in blue. It could be seen that the lines of force  clash in between the turns. Then they cancel and those areas become ‘null points’. Shown by the letter ‘n’. The forces join at the axis of the coil. This strong magnetic field is shown in green. If you insert a soft iron into the coil the strength of magnetism will increase as the lines prefer to pass through iron rather than through air.

Poles of the Electromagnet.

The side from which the lines emanate is the North pole and at the South pole the lines go in. There is a very easy way to find this.  It is called the Fleming's Right hand rule.


Fig.5. Fleming's right hand rule.

Grip the coil so that your fingers will point the direction of the current. Then the thumb will point the North pole. 


 Fig.6. Use the right hand rule to verify  the N and S poles.

Electric Bell

  The Electromagnet is shown in grey.  When the current passes it attracts the steel strip which acts as a spring. This is fixed at the base. Then the striker hits the bell. This will cut off the contact of the strip with the screw. As magnet does not get current the steel spring will pull the striker back This can hit can hit another bell. So on and so forth it will go on ringing as long as the current supply is there. 

Fig.7. Electric bell.