Current Balance: Lab #10

Current Balance: Lab #10


Name ______Date ______


Current Balance: Lab #10

M.L. West, Sargent-Welch Scientific Co., Eduquip-Macalaster Corp.

Objective: to use a current balance to determine the relationships between force,electric current, and magnetic field.

Equipment:current balancing blade,air core solenoid,DC power supply (up to 5 amps), rheostats, multimeter, thread, scissors, digital balance, meter stick, Welch current balance

Two Sketches:

Background:When a current carrying wire interacts with a magnetic field there is a force between them.

When two current-carrying wires are near each other they exert a force on each other through their respective magnetic fields.The official MKS definition of one ampere is “that unvarying current which, if present in each of two parallel conductors of infinite length and one meter apart in empty space, causes each conductor to experience a force of exactly 2 x 10^-7 Newtons per meter of length.”


The Welch Current Balance apparatus (part C) must be shared, so plan accordingly with other lab teams.

A. Current in the Wire

1. Weigh thread with a length of 2 x number of lab groups. Calculate the thread’s mass density per meter.

Each lab team will need 2 m or 10 chunks of .2 m each. Cut the thread carefully into these .2 m chunks.

2. On the balancing blade measure the length L of the end wire segment, and the distance d2 from the segment to the balancing pins. Measure the distance d1 from the balancing pins to the place where the string will be put.

3. Attach the support clips for the balancing blade to the front face of the coil with its terminals near the table. Be careful that the support clips do not touch the terminals for the coil itself. Place the blade gently on the support clips so that the segment L extends deep into the coil, and the free end of the blade extends over the edge of the table..

Use the meter stick to measure its equilibrium distance from the floor.

4. Connect the power supply to the air core solenoid through an ammeter.

Connect the power supply to the balancing blade through another rheostat and ammeter (or use a separate power supply).

Care must be taken to stay within the rated current capacity of the power supply. If you find that a current beyond the rated capacity is required to balance your blade, then STOP.

Adjust the power supply to obtain a current of 1.5 amps to the coil. You will keep this constant for this part of the experiment.

Turn on the current to the blade briefly to check that the blade tips downward inside the coil.

5. Gently drape a chunk of thread over the tab at the outside end of the blade.

Turn on the current to the blade L and adjust it until the blade is restored to its equilibrium position. Record this current.

Repeat with up to 10 increments of thread or until the current reaches 5 amps.

B. Current in the coil:

6. Keep the current in the wire of length L constant at 2 amps.

Vary the current Ic in the coil to rebalance various increments of thread. This will allow you to investigate the relationship between the current Ic and the B field it generates.

C. Current Balance Apparatus:

7. Orient the apparatus so that the parallel wires run north-south. (This will eliminate the effect of the Earth’s magnetic field.)

Level the apparatus on the table.

8. The current to be measured passes through the two parallel horizontal bars. The lower one is fixed in place, but the upper one can move slightly.

Connect the two wires in series. Which way is the current flow in each wire?

9. Note the equilibrium position of the upper wire.

Put a small weight onto the groove pan on the upper bar.

Turn the current on so that it repels the upper bar, and returns it to its equilibrium position.

Repeat with up to 10 increments of thread or until the current reaches 5 amps.


A. Plot current I through the wire length L vs. string mass. Fit a linear trendline and calculate B, the magnetic field of the coil, using

Gravitational torque = m g d1 = current torque = I L B d2

B. Plot current Ic through the coil vs. string mass.

Plot B vs. Ic. How does B depend on Ic?

C. What should you plot in this case? The force should be proportional to length/separation and also to some power of current. Do some exploring, and comment on the results.

Discussion and Conclusions:

Compare the two methods as to ease of use and precision.

How might this experiment be extended or improved?