User:Jake Mokris/Hall Effect

Hall Probe Adjusting Knob

So, this is what's going on: the samples are attached to a plate that you insert in the magnet. You connect all the detectors to this plate. Now, at the top of the plate, there's a knob that's supposed to adjust the resistance of the circuit that connects to the sample (this is from the manual):

"Before exposing the Hall effect apparatus to the magnetic field, adjust the zero point: Apply a transverse current I of e.g. 10 A and set the indicator of the meter for measuring the Hall voltage UH to zero using the adjusting knob. If the display changes after switching off, switch the transverse current back on and repeat the zero-point adjustment."

It turns out that this knob doesn't work. Depending on the position of the knob, it is possible to break the circuit entirely, so that no current will flow through it. Prof. Marriage measured the resistance of the circuit while varying the position of this knob, and found that at certain positions the resistance was infinite. This is clearly bad, for if the circuit is broken, the measurements of the Hall voltage won't change. And this is precisely what I've seen several times while taking data: the reading on the microvoltmeter doesn't change, even if I alter the magnetic field significantly.

What to Do About This

Before you take any data, measure the resistance across the sockets to which you attach the microvoltmeter. Turn the resistance-adjusting knob so that the resistance of the circuit is not infinite; the resistance should be very small. Leave the knob in that position while you take data.