This is an amazing piece of equipment. The Earth's magnetic field passes through the wooden circle, which contains a big coil of wire. Flux from the Earth passes through the coil. Hence there is flux linkage - which is defined as number of turns on the coil x flux. There is a handle at the top which I managed to cut off in the photograph. If I rotate the handle, the wires spin in the flux. Imagine a quarter turn through 90 degrees from full flux linkage with all of the flux passing through the coil to a point at which the coil is parallel to the flux and no flux passes through the coil. Faraday's Law states that induced emf = change in flux linkage/time taken. An emf is a voltage which could be measured by attaching a multimeter to the contacts. Flux changes from N x flux back to zero in time delta t. Delta t is one quarter of the time period so it is T/4. So emf = 4N x flux/T. I rotate the coil at a steady rate and time 20 cycles. I then diivde by 20 to get T. Finally, the Earth's flux = emf x T/4N. If I want magnetic flux density B, then B = flux x area of the circle.
