HLab 12 (online).  Calibrating a Hall Probe

Goals

• To use a Hall probe to sample the field of Helmholtz coils
• To determine the calibration curve for a two-axis Hall probe in the radial mode

Equipment
HE1-A Hall Probe
Set of Helmholtz coils
2 Multimeters
Constant current supply
Hookup wires
Ruler

Introduction
The Hall probe's output signal is a voltage that is proportional to the strength of the magnetic field in which the probe is placed. The calibration curve is a function that relates the Hall voltage to the strength of the field. Knowing the calibration curve, you will be able to use the probe to measure the strength of an arbitrary magnetic field. The two-axis Hall probe actually contains two probes oriented perpendicular to each other, so that the device can be used to measure two independent components of the magnetic field at a single location. You will determine separate calibration curves for each of the two probes.

In order to perform an absolute calibration of the device, you must put the probe in a known magnetic field. The best available source of a known magnetic field is a set of Helmholtz coils. This configuration is used to create a nearly constant magnetic field in the region around the point midway between the coils.

Method
Work in a group of 2 or 3 people. All work must be done as a group.  While you work, check each other out to make sure the probe is being positioned correctly and the meter is being read correctly.  You'll enter some results online, so you'll need to have this page open on at least one computer while you're working.  Enter all partners' names now.

Partners' names:

Give your group a memorable name for future identification (for example, quarks, but make sure no other group duplicates your name):

Your coil set and hall probe each have a serial number.  Record those now.

Serial number of coils:    Serial number of probe:

The following equipment specifications are given by the manufacturer.  You'll need these in calculations later

• Number of turns per coil, N = 168
• Radius of either coil, R = separation of planes of coils, S = 0.070 m

About the current supply: The current through the Helmholtz coils must not exceed 3 Amps, or the coils may overheat and cause damage to the apparatus. The power supply should be operated in constant current (CC) mode to ensure that the magnetic field of the coils does not change with time. The red light underneath the power supply's ammeter will be lit when in constant current mode. If the red light goes out (and a green light under the power supply's voltmeter turns on), adjust the coarse voltage dial until the red light reappears. You may then need to readjust the coarse current dial to obtain the constant current you want.

Compose answers to the questions below as a group.

Part A. Using the Hall probe to sample the field

Don't turn on the power supply until after the instructor checks your wiring.

1. Wire the coils in series with the current supply so that the magnetic fields of the two coils add constructively.  A positive current entering the red terminal of either coil will produce a magnetic field vertically upward along the axis of the coil. Include an ammeter in the circuit for measuring current.  Set it to the 10 A scale. (You may be able to dispense with the ammeter if you're using one of the new power supplies with digital readout.)

Explain why it's better to wire the coils in series instead of parallel.  (The answer has to do with the magnetic field that you need to set up.)

From the perspective of a person looking down on the coils, what direction does conventional current flow in the coils?

clockwise
counterclockwise

1. The Hall probes are located near the end of the long tube on the device. Set up the Hall probe so that the tube is perpendicular to the coils' axis, and the end of the probe is positioned midway between the coils.  Also make sure that the control side of the Hall probe control box is oriented vertically.  (Ask the instructor if you're not clear what this means.)  Set the sensitivity of the probe to x10 and toggle to the transverse mode. The probe will then measure magnetic fields in the direction of the coils' axis (transverse to the direction of the tube).
2. The current must remain off for this step. Attach the multimeter leads to the Hall probe terminals and set up the multimeter to function as a voltmeter. Plug in the Hall probe and adjust the zero adjust on the probe until the output voltage is zero. Then switch the probe to x1 mode.

Why is it necessary to perform the zero adjustment?

Why is the x10 setting used for the adjustment?

1. Make sure that the Lo/Hi button on the power supply is set to Hi and all knobs are turned all the way counterclockwise (zero setting). Now turn on the power supply and adjust the current to about 2 amps. There are 2 scales (on the analog power supplies). Make sure that you're reading the correct one. You'll need to adjust the voltage in order to keep the power supply operating in constant current mode. (See note above.) Observe the output signal of the Hall probe.
2. Sample the field vertically by adjusting the probe up and down while watching the voltmeter.  Return the probe to the center of the coils.  Then sample the field horizontally by adjusting the probe back and forth while watching the meter.

Describe the results of your sampling procedure. How sensitive is voltmeter reading to the position of the probe?  In other words, how uniform is the field?

Part B. Calibrating the Hall probe

1. Orient the coils so that you can insert the Hall probe into the coils along the axis. (You'll need to turn the coils on their side for this.)  Devise a method to determine the calibration curve for the Hall probe, i.e. the function B(V) where B is the magnetic field along the coils' axis and V is the Hall voltage. List your method in stepwise fashion (1,2,3, etc.) clearly and completely below. (Note that in order to complete a calibration, you need to compare to a standard.  In this case, the standard can be the values of magnetic field that you calculate using the formula for the field of Helmholtz coils.)

Continuing Part B...

1. Switch the Hall probe to radial mode and re-zero it.
2. Carry out your calibration method, recording your measurements in Graphical Analysis as you take them.  Give your column headings descriptive names for the variables with units and set the number of decimal places to the appropriate values.  If you don't know how to do this, ask.
3. In a textbox, write the name of your group.  Write the serial numbers of your coils and probe.  Also write the equation that you will use to calculate the theoretical field magnitude of the coils.  See your result from the Helmholtz coil homework problem.
4. Create a new, calculated column for the values of magnetic field calculated using the equation from 4.
5. When finished taking your data, turn off the current supply and disconnect all wires.  Have the instructor check your station.

Analysis

1. In Graphical Analysis, plot B vs. V.  Perform a linear fit.  Title your graph properly and by all means remove the connecting lines.  If you don't know how to do this, ask. In the text box, rewrite the fit equation in terms of physics variables B and V, and the values of coefficients with units.
2. Save your file with the name hlab12-groupname.ga3 and email it to your instructor.
3. Email copies of the GA file to all group members.