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Objective of Experiment. To use a search coil and CRO to investigate the magnetic field due to a straight wire carrying an alternating current

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7A (27)

Magnetic fields around electric currents I – straight wire


To use a search coil and CRO to investigate the magnetic field due to a straight wire carrying an alternating current


Search coil, lateral and axial


Signal Generator


Low voltage power supply

PVC-covered copper wire


Slotted base

Perspex strip

Crocodile clips

Connecting leads        


The magnetic field due to the electric current in a long straight wire is such that the

field lines are circles with the wire at the centre.


The experiment should show that the magnetic field B at a point near a long straight wire is directly proportional to the current I in the wire and inversely proportional to the distance r from the wire.


This relation is valid as long as r, the perpendicular distance to the wire, is much less than the distance to the ends of the wire (i.e. the wire is long).  

...read more.



Signal generator frequency f = 5000Hz                

Length of vertical trace on CRO l/cm






Current I/A
















Length of vertical

...read more.


In addition, there are errors produced by moving the coil to investigate relation between r and l. This is because the coil is not moving along the same line when varying the distance r from the wire. Also, reading error is engendered when reading the mark of ruler, trace of CRO and ammeter. Furthermore, there were other electrical appliances which produced changing magnetic field and the result of the trace of CRO thereby was affected.

Improvement could be made to increase the accuracy of the experiment. First, the sensitivity of search coil should be increased by increasing the frequency.The more frequent the voltage change across the wire, the more frequent is the change of the magnetic field. The magnetic field across the search coil is proportional to the sensitivity. Second, the search coil should be rotated to obtain the maximum value of the magnetic field to ensure the field is perpendicular to the coil. Third, the set-up should be placed well away from the stray fields such as those from main sockets.

...read more.

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