Linear Trend Develops Between Frequency and Required Field Strength for Levitation

I was greeted by a hissing liquid nitrogen Dewar when I got to the lab today. Apparently it's normal Dewar venting, but our last two didn't do this, so it was a bit interesting. I was able to take more data today and there's a linear trend evolving. As the frequency of the current driving the electromagnet is increased,the amplitude of the current has to be increased to achieve levitation of the superocnductor. A picture of theresulting graph is shown below. The x axis is frequency. The y axis is the peak voltage detected on the pick-up coil wrapped around the electromagnet.

Here's a picture of the modified pick-up coil.

It's just three windings around the top of the magnet.

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