In this experiment, we examine the direction and magnitude of magnetic fields. The direction of the magnetic field can be determined using a small compass needle. At each point, a freely rotating magnetic compass needle will settle pointing along the magnetic flux line, so that by plotting the direction of the field, we can trace lines of magnetic flux. We will examine the magnitude of the magnetic flux density along the axis of a circular coil using a magnetic field sensor connected to a computer.  

(a) Magnetic field lines from a bar magnet

The magnetic field of a bar magnet is plotted first. The bar magnet is placed in a slot in the apparatus and can be seen in the image above as the grey horizontal line on the blue base. The coil above is not being used in the part. An A3 sheet is placed over the blue base and magnets are placed at various point on the sheet. The direction of the magnetic field is read off the compass and marked on the A3 sheet. This is repeated all over the sheet to plot the magnetic field of the bar magnet. 

You can try a similar experiment here: Magnetic Field Interactive

 (b) Magnetic field on the axis of a circular coil

The apparatus is then set up as can be seen above and  the bar magnet removed from its slot in the apparatus and place well away from the coil before doing this part of the experiment. It has already been discussed in a previous experiment that a current flowing through a wire produces a magnetic field. The circular object protruding from the base is a coil of wire where the wire has been coiled around the circle multiple times. The power is then turned on and the current plotted on a new sheet just as done above. With a current of about 0.8 Amp in the coil, its magnetic field over the page is comparable to the earth’s magnetic field, so that the field lines on the page will be substantially affected by the orientation of the coil with respect to magnetic north.

Using the computerised magnetic field sensor, the magnetic flux density may be measured at a series of points (say, 1 cm apart) along the axis away from the coil and can be compared to theoretical values. 

 You can try a similar experiment here: Magnetic Field Along The Axis of A Circular Coil Carrying Current