Experiment 10
THE MAGNETIC FIELD

PROCEDURE


The apparatus includes a pair of Helmholtz coils, shimming block, 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 5 A), reversing switch, and a 22 Ω (5 A) rheostat.


PART 1: Calibration of the Jar Magnet

1.  Open the Worksheet and fill in the header information.


Figure 10-3: Circuit for the Helmholtz Coils


2.  Connect the circuit as shown in Fig. 10-3. 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 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. Turn the power supply off.

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 with the jar magnet 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. The period of oscillation is equal to the elapsed time in seconds divided by 50. Record the data in cells C45:D45.

6.  Repeat this measurement for coil currents of 1.0, 1.5, 2.0 and 2.5 amp. Record the elapsed time and period data in the Data Table covering cells C46:D49. When you have completed taking the data, turn the power supply off.

7.  Record the characteristics of the Helmholtz coils: the number of turns and the diameter, in cells D53 and D55.


PART 2: The Earth's Magnetic Field

1.  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. Record the time in cell C64.