Introduction In the lab session, the repulsive magnetic force between two wires with antiparallel currents was balanced by a known gravitational force. The result obtained from the lab session was used to calculate an experimental value for the permeability of free space µ o (Lehn 1151). Special Concerns for the Lab The lab required extremely careful manipulation of the sensitive balancing information. Excessive movement in the Lab room was thus discouraged. The currents were left ON for 60 seconds or less since the large currents used in the current balancing experiment could cause excessive heating. Additionally, the output signal was always brought to a minimum first before turning the power supply off or on, by rotating the voltage adjusting the knob counterclockwise to the end position. Objectives   * To determine an experimental value of the permeability of free space µ o   * To investigate the balancing of repulsive magnetic between two wires with antiparallel currents by using a known gravitational force Theory/Literature Review The setup for the current balancing experiment can be shown in figure 1 below. Figure 1 the current balance apparatus. The current I can be passed in opposite directions through the horizontal bars. The lower bar can be fixed in place, and the upper bar is movable and can be brought to a few millimeters of the fixed bar using an adjustable counterweight. The gap between the bars can be measured by an optical lever and should be set to be approximately 2.0mm with the current off. A set of masses are attached to the upper bar. The current I can be slowly increased until the movable bar returns to its initial position to compensate for the added weight. At the time the repulsive magnetic force is approximately equal to the weight of the added mass. The measured magnetic force can be equated with the calculated magnetic force F B using the relationship: F B = µ o LI 2 /2πr Where L was the length over which the currents were antiparallel and r represents the separation distance between the two currents. Equipment   * Power Supply 30V   * 1Ω resistor   * Voltmeter   * Fixed bar movable bar   * Pan mirror   * Connecting wires   * Ruler   * Optical lever Procedure Masses were added to the pan starting with 20mg ad going to 120mg in units of 20mg. For each mass, I was adjusted so that the upper bar could return to its equilibrium position. M and I were recorded. The masses were removed in units of 20mg, recording m and the necessary I for equilibrium. When all the masses were removed, the equilibrium position was checked not to have changed by the action of adding and removing masses. The results were affected by the magnetic field of the earth. To reduce the effect, a two-way knife-edge switch was used to reverse the current direction and the entire steps above were repeated. The value of µ o for each experiment was calculated and the average for the two values was computed. To determine the values of µ o from the two experiments, a graph of I 2 versus mg was drawn, first for the forward current direction, and again for the reverse current direction. Two values of µ o were determined from the slopes of the graphs. The average of the two values was computed to get the final experimental va