Monday, July 2, 2012

Measuring Magnetic Field vs. Frequency Using a Transformer

Alternating current power can be reflected by a reactive load, (like an electromagnet). Ham radio operators are familiar with this concept and measure the amount of reflected power as the standing wave ratio, (SWR). As the frequency driving a ham radio antenna is changed away from the antenna's resonant frequency, the SWR increases, less power is driven into the antenna and more power is reflected back into the radio's amplifier.

On Friday, I observed that the levitation height of the superconductor decreased when the frequency of the current driving the levitating electromagnet was increased. This is the expected result based on the experience of the other teams that tried to replicate Podkletnov's experiment. However, the same behavior would have resulted if power from the amplifier was being reflected without entering the electromagnet.  The amount of reflected power is usually measured with a directional wattmeter, or with an SWR meter.  I don't have either of those instruments.  While I could measure the voltage on the terminals of the electromagnet, that would only give me information on the standing wave at the terminasl, not necessarily how much of the wave was being accepted by the magnet.  Current that was driven into the electromagnet's create a magentic field, while current that was reflected would not.  So, what  I needed was a way to measure the magnetic field produced by the electromagnet.  I have a Gauss meter sitting on my bench, but it only works for DC magnetic field.  The AC magnetic field averages to 0as it changes direction several hundred times per second.

The trick to measuring the current delivered was to use a pick-up coil, (or transformer), to measure the magnetic field produced and then read it as a waveform on the oscilloscope.  Faraday's law of induction states that a changing magnetic field will create a proportional changing electromotive force.  If a coil of wire is placed in the region of the changing magnetic field, (and therefore the electromotive force), a current will flow in it.  Using a spare coil of wire that was laying around, I setup the measuring instrument:

The coil of wire is sitting directly on top of and aligned with the electromagnets pole piece.  The two red circles highlight where the oscilloscope probes connect to the two leads of the coil of wire.  Using this simple instrument, I was able to get oscilloscope waveforms that I could use to check the dependency between the magnetic field produced, (proportional to the peak value of the waveform), and the frequency fo the current drvining the electromagnet:

There was no variance between the peak value of the waveform and the frequency of the driving current.  At these frequencies, all the current is getting into the electromagnet.  This indicates that the observed variation in levitation height was due to change in frequency of the magnetic field and not due to a change in its amplitude.

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