To determine the variation of magnetic field with the distance along the axis of a current carrying coil, follow these steps:

1. Obtain a current carrying coil and a Gaussmeter or a Hall effect sensor to measure magnetic field.
2. Place the Gaussmeter or Hall effect sensor at a fixed distance from the center of the coil and measure the magnetic field.
3. Repeat step 2 for different distances along the axis of the coil.
4. Plot the magnetic field vs distance graph to see the variation.
   
   

To estimate the radius of the coil, follow these steps:

1. Measure the magnetic field at the center of the coil.
2. Measure the current flowing through the coil and the number of turns in the coil.
3. Use the formula for magnetic field due to a solenoid to calculate the radius of the coil.
4. Compare the calculated radius with the actual radius to determine accuracy.