Distance relationship between primary/secondary coils

[tima89]

We have done a simple experiment on AC transformers.
Given: AC Supply. Big primary coil. Smaller secondary coil. Even smaller metal rod. (They all fit into each other) Voltmeter
What we did:
Connected AC supply to the primary coil. Voltage kept constant at all times
Secondary coil was placed in the primary coil, and connected to a voltmeter.
procedure:
Secondary coil was raised X distance from the primary coil. (Measured top to top).
Voltage on the secondary coil measured with each step.

Now, our results (Voltage vs. Distance) showed parts of a sin (or cosine) graph, and we are having a hard time justifying it. (i know that flux is BAcos(theta)
I would be very pleased if someone could explain it.

Thanks in advance

 

[berkeman]

What equation should govern the pickup voltage as a function of distance? What equation describes the 3-D shape and intensity of the magnetic field created by a wire loop?