Experiment 17



Calibration of the Jar Magnet

The apparatus includes a pair of Helmholtz coils, a cardboard template with cutouts for the coils, shimming blocks, iron filings, a small compass, a jar magnet (a small bar magnet about 5 cm long by 5 mm diameter with one end painted, which is suspended from the lid of a glass baby food jar by a nylon thread), a dc power supply (up to 5 amp), a timer, an ammeter (0 to 5A), reversing switch, and a 22W (5A) rheostat.

1.    Open up the Magnet Workbook and fill in the header information.

Figure 17-3: Circuit for the Helmholtz Coils

2.     Connect the circuit as shown in Fig. 17-3. With the output knob of the supply turned fully counterclockwise (CCW), turn on the supply and increase the output until the ammeter reads 0.5 amps.

Take the compass and determine the field direction at several points inside and outside the coils. Recall that the North-seeking end points in the direction of B. Make a sketch of some of the field lines produced by the Helmholtz coils in the indicated space on the Worksheet.

3.     Reverse the current by flipping the reversing switch and repeat the above procedure. Sketch the field lines in this case and note any differences in the space indicated on the Worksheet. Turn the power supply off, remove the cardboard template and replace the iron filings in their container.

4.    Place the jar magnet in the center of the Helmholtz pair at a horizontal level that is close to the longitudinal axis of the coils. Determine magnetic North by using the jar magnet as a compass, and rotate the Helmholtz coils so that their axis points in that direction.

5.    Turn on the power supply and set the current at 0.5 amp. Measure and record the time for the jar magnet to perform 50 oscillations in cell C44. The period of oscillation is equal to the elapsed time in seconds divided by 50. Record the period in cell D44 on the Worksheet.

6.    Repeat this measurement for coil currents of 1.0, 1.5, 2.0 and 2.5 amp, and record the values in cells C45:D48. When you have completed taking the data, turn the power supply off.

7.    Enter the number of turns in the Helmholtz Pair in cell D52. Measure the diameter of the coils in meters, and enter the result in cell D54.

The Earth's Magnetic Field 8.    Set the jar magnet as far as is practicable from any steel or iron items (especially the steel girders that may be in the walls). Measure and record the time for 100 oscillations in cell C63. Determine the period of the oscillator by dividing the previous reading by 100, and enter in cell D63 on the Worksheet.

9.    Print out the Worksheet and use it to develop your lab report.