Measuring magnetic field strength by direct force measurements
Here is my attempt at measuring magnetic field strength. To measure the field generated by the electromagnet above I made a coil that could be placed in the magnetic field.
“Continue reading” for measurements and results. 3 photos and links.
A known current can be passed in it, the force measured and hence the field calculated from the formula:
B (Tesla) = Force (Newtons) / i (current amps) x length (meters).
Above: The coil is 10 turns with diameter and has a diameter of 3 cm. It looks like the voice coil out of a loudspeaker and is being used for a similar purpose. Hence length is 0.03 x pi x 10 turns = 0.94 m. The magnet is not touching the voice coil. The balance arrangement with the meter reading 4.78 g with 100 mA running in the voice coil with the NIB magnet. The fulcrum arrangement is such that the meter is at 16.5 cm and the voice coil is at 37.5 cm. Hence:
actual force on voice coil is 4.78 x 16.5/37.5 = 2.10 g
Force is measured in Newton’s in formulae: 1 Newton = 102 g
Hence voice coil force = 0.020 N
Using the formula above:
B (Tesla) = Force (Newtons) / i (current amps) x length (meters)
B Tesla) = 0.020 N / 0.10 A x 0.94 m
= 0.21 Tesla
This seems too low for a magnet I would expect to be up to 0.5 – 1.0 Tesla. Was it heat damaged or is my technique incorrect?
Let’s try a different magnet.
Above: The voice coil sits in the big electromagnet gap. The balance arrangement with electronic scales with the meter reading 0.70 g with 100 mA running in the voice coil. Actual voice coil force accounting for the fulcrum is 0.31 g. Note that this is much smaller than the NIB magnet . The electromagnet is on with 43 V at 25 A = 1075 W and doesn’t even get warm for short runs.
B (Tesla) = Force (Newtons) / i (current amps) x length (meters)
B (Tesla) = 0.020 N / 0.10 A x 0.94 m
= 0.031 Tesla
Again, this seems tiny. I will make up a Hall effect gauss meter to compare magnetic field strength. No doubt this electromagnet can take a lot more power in the short term such as 100 A instead of 25 A but this still only makes it 50% of a NIB strength for 5 kW input. Admittedly, the active area of concentrated field is much greater between the two poles.
I need to get some magnetic field strength readings with my new Tesla meter.
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Photo Date: 2006