The current balance is used to measure the force of repulsion between identical oppositely directed currents in parallel conductors. The magnitude of this force can be shown using classical electrodynamic theory to be , where I is the current in either conductor, L is the length of the conductors, r is their separation and m ₀ is the magnetic permeability of free space.

• Pasco EM-8623 Current Balance, which includes:
• Instruction manual and experiment guide
• Current balance apparatus
• 9V transformer
• 20 N spring scale
• 500 x 1 mg mass set
• Compass
• 0.6 ml vial of gallium
• Allen wrench
• Power supply for current
• Calipers

First you must set up the equipment. Read the Pasco manual to learn how to do this. Be sure that the heater for the gallium contact points is plugged in, otherwise the gallium may solidify and you could damage the apparatus.

Use the small compass to orient the parallel conductors to minimize the effect of the earth's magnetic field. The balance must be zeroed and a calibration factor for the torsion wire must be determined, i.e., a force/degree factor must be determined. Test the torsion wire carefully, as over time it may have been stretched or kinked, and may no longer respond properly with the turning of the dial. If this is the case, you will have to use small masses to provide the balancing force.

Determine the current dependence of the force between the two conductors, the dependence on separation, and the permeability constant m ₀ for your chosen set of units. Use the calipers to measure the separation between the conductors. Determine the magnitude of the earth’s magnetic field.

Follow the procedure described in the "zeroing" section of the manual. Use the graduated pitch screws on the lower conductor to set the conductors at a known separation. Increase the current in small increments and use the torsion wire (or masses) to return the balance to zero after each increment. Be sure not to exceed a current of 15 A. Determine the magnetic permeability constant m _0.

It will be to your advantage to calibrate and use the torsion wire if you have not already done so. Set the current to a value between 5 and 10 A and keep it constant for all measurements here. Vary the separation between the conductors by adjusting the lower conductor. Record the force necessary to return the balance to zero for a range of separations. Use your data to determine both the permeability constant m ₀ and the diameter of the conductors (assume identical conductors).

Bypass the lower conductor so that the current only flows through the balance. Orient the balance to obtain maximum deflection by the earth's magnetic field. Measure the force required to zero the balance for as large a current as possible (maximum 15 A !). Determine the strength of the earth's magnetic field in 309 Osmond. Have you determined B completely, or just component of it?