Magnetic Field Measurements

A. Calibration Procedure

1. Apply a direct current of 0.4 A to the Helmholtz coils. Use the ULI magnetic field probe set on high amplification with LoggerPro. Pay particular attention to the directional sensitivity of the probe. Decide whether to use the high or low sensitivity of the probe.


2. Calibrate the magnetic field probe by inserting it into the center of the coils. Rotate the probe to align the white dot with the coil axis. Enter the calculated magnetic field value. The magnetic field in Tesla produced at the center of the Helmholtz coils is

where N = 124 is the number of turns in one of the coils, I is the current in Amperes, and R is the radius of the coils in meters. Now rotate the probe 180 degrees to align the dot exactly opposite the original direction. The calculated field should be the negative of the value previously entered. Enter that value.


B. Field Mapping


1. Position the probe at the center of the coils. Starting with the white dot on the probe pointing along the axis of the coils, rotate the probe 360o in 90o segments. Interpret your results.


2. Starting at the center of the coils, measure the axial component (dot parallel to the axis) of the magnetic field at a number of points a.) along the axis of the coils and b.) perpendicular to the axis of the coils. Note that in both cases the white dot should be pointing in the direction of the coil axis. Be sure to include points outside the coils. Plot the peak magnetic field vs the distance from the center of the coils for both a and b. Be sure to determine the extent of the region of uniform field. Compare the measured value for the peak field at the center with the expected value.