Advanced Lab - Hall Effect:Hall Effect Experiment

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2. Hall Effect Experiment

This part describes the Hall Effect apparatus, as well as the procedures for computer control and data collection.

2.1 Apparatus & Equipment List

Figure 1: The Hall Effect Apparatus.

Figure 2: Hall Probe Detail

Equipment

Alpha water-cooled magnet and power supply (Turn on water before turning on power supply. Do not exceed 10 A magnet current).
Hall Probe, Indium Arsenide (InAs), red and black current leads, green and white Hall voltage leads, installed in oven, with stand.
Extra Hall Probe for examination, with stand.
Oven, to heat Hall probe.
Digital thermometer with thermocouple and display.
DC Power Supply to furnish current to Hall probe; adjust only the "voltage" control; probe current not to exceed 0.4 A.
Variac to control AC current to oven; blocked to restrict temperature to 100°C.
Ammeter, to measure Hall probe current.
Voltmeter, high input impedance, to measure Hall voltage.
Magnetic Compass, to determine direction of magnetic field.
Gaussmeter to measure magnetic field.
Hall effect probe and calibrated control unit with stand for probe.
Computer and data acquisition board.

2.2 Precautions

Personal

The magnet power supply can furnish large currents at dangerous voltage levels; do not touch exposed magnet coil contacts.
The oven gets hot.
AC leads from Variac to oven can be dangerous; they should not be exposed.

Apparatus

Never suddenly interrupt or apply power to a large magnet. Large inductive voltage surges may damage the insulation. Start with controls set for zero current and gradually increase current. When turning off, smoothly decrease current to zero and then turn off.
Turn on water before turning on magnet coil.
Do not exceed magnet current of 10 A.
Do not exceed Hall probe current of 0.4 A
Do not exceed an oven temperature of 100°C (a few degrees more for a brief time will do no harm).
Do not leave the magnet current at a high setting for any length of time beyond the minimum needed for data acquisition - it affects the monitor (obviously).

2.3 Procedure

The steps listed below outline the procedure for taking the data using the computer to investigate both the Hall and magnetoresistance effects. Complete sets of data should be taken at three temperatures: room temperature, between 65-70°C, and 100°C. The Notes Section contains a figure that will assist in finding the appropriate variac settings. The temperature needs to reach equilibrium before you begin taking data.

Data Steps for Collecting Hall Effect Data:
Measure the Hall voltage (VHall) for each of four probe currents (0.1, 0.2, 0.3, and 0.4 A) for each of five magnet currents (1,3,5,7, and 9 A). At each magnet current, use the gaussmeter to determine the magnetic field. In order to make the field measurements, the Hall probe - oven - thermometer assembly must be removed and replaced by the gaussmeter probe. The Hall probe-oven assembly is not very sturdy, handle it carefully.

Data Steps for Collecting Magnetoresistance Effect Data:
For each of the five magnet currents listed above and zero magnetic field strength (remove the Hall probe from the magnet to minimize any stray fields), measure the resistance of the Hall probe.

Procedure for Controling the Apparatus and Collecting Data Runs

Before doing anything, turn the cooling water on so that the magnet does not over heat!
Turn Computer on. (the password for the login is "taisgood").
Run the program Hall Effect shown on the desktop.
Set the temperature to desired amount using the Temperature Variac. Don’t forget to write this value down.
Make sure "Magnetic Current" on the computer screen is set to zero amps.
On the Power Supply for the "Magnetic Current", flip the switch to "on".
Both Screens on the "Magnetic Current" display should read zero (or something very close)
Back to the computer: With the mouse, point to a button in the upper left-hand corner of the screen (a little below the word "File" in the menu bar) that has a pop up balloon that says "Run Continuously." Click on this button.
Wait for the error screen. Click "Continue."

Note: this error message doesn’t mean anything important, so don’t worry about it.
Click once on the "Do it!" button so that the green light on the button lights up.
Raise Magnetic Current to your desired amount of amps. (don’t forget to write down what the value for the Magnetic Field is off of the gaussmeter) (You’ll need this later) (also, note the units on the gaussmeter. The Analysis portion of the lab calls for them to be in tesla)

Note: Watch the right screen on the Power Supply for the "Magnetic Current" rise as you increase the Magnetic Current on the computer screen. This is obviously the current going through the magnet. Don’t raise this too fast or past 9 amps because the Power Supply will shut down and lock up. If this happens, turn the Power Supply off for a moment and go back to step number 5.
Go to the "Get I-V Curve" menu screen and set the parameters for the experiment.

Set the Current Max to 0.4A

Set the Current to step up in increments of 0.1A

Set the number to average the experiment to 20 (the higher this number is, the more accurate the data, but the slower the experiment will run...20 is a good number).
While still displaying the "Get I-V Curve" menu screen, click on "Do it!" twice so that the light goes off then comes back on.
Wait while the computer runs the test.
Watch the red "Probe Current and Voltage" display (where the red and black wires are connected to) as the computer runs the test. Notice how the program pauses at each step of current and takes the specified number of measurements at that amount of amps. Also watch as the Hall Voltage readout changes (where the white and green wires are connected to) as the computer runs the test.
After the program has ran it’s test, save the file to a location in the computer.

(When you run the file to do the analysis part of the lab later, it should be opened in Microsoft Excel or some other program of similar type that will put the data into tables and columns. Once opened, four columns of data with five rows each will be in the saved file:
- The first column is the Hall Voltage in volts.
- The second column is the corresponding Probe Current in amps. It should read from 0.0 to 0.4 in steps of 0.1.
- The third column is the resistance across the Hall Probe in Ohms, which is the voltage across the probe divided by the probe’s corresponding current (column 4 divided by column 2).
- The fourth column is the voltage across the Hall Probe (the Probe Voltage) in volts due to the current carriers (ie. the voltage on the red digital read-out from the “Hall Probe Current”).
Click on the red stop sign (the "Abort Execution") in the top left menu bar (next to the "Run Continuously" button from step 6). If this is not done, the experiment keeps running continuously and will ask multiple times to save data.
Congratulations, you’ve run your first test.
Go back to the first menu screen.
Go back to step 8 to run another test, setting new temperature, magnetic current, or both to desired values.

2.4 Notes

The gaussmeter zero needs to be monitored and adjusted as needed. To do this, remove the gaussmeter probe well away from the magnet and check the zero for the scale that is in current use. The zero may shift when the scale is changed. To detect the effect of a stray field, observe the zero as the probe orientation is change by about 90 degrees.
During the experiment investigate the geometry of the set-up. Note the direction of the magnetic field, probe current, and Hall voltage – this will help in the analysis. The extra probe is available to help with this (see also Figure 2).
Use MKSC units consistently. Magnetic fields should be expressed in Teslas (T = Ns/mC = kg/Cs). The physics literature often expresses magnetic fields in gauss, with 1 T = 10,000 gauss. Pay attention to the units on the gaussmeter. Always record data in the same units as used by the measuring instrument, but with the units noted on your data sheet. Make necessary conversions later.
Ohm = Js/C²; units of conductivity are (ohm-m)^-1.
Use Figure 3 to help determine the correct Variac setting for the desired probe temperature:

Figure 3. Probe Temperature vs. Variac Setting