Measuring the Magnetic Field of a Slinky

A simple way to generate a magnetic field is to run an electric current through a wire. For a long straight wire we can write down a formula for the field,

B = μ_o i / 2 π r

This makes an assumption about the geometry of the situation, namely that the wire is infinitely long. Wires (and therefore currents) with different geometries will generate different magnetic fields. Our goal for today is to measure the magnetic field at the center of a Slinky, and to determine an appropriate formula.

Why do we pick a Slinky? We make this selection for several reasons. With a Slinky we can quickly change many variables. From our knowledge of the straight wire, we should expect that the field varies with the current. We can also see if length plays a role in the strength of the magnetic field simply by stretching the toy. By moving the clips that connect the Slinky to the rest of the circuit, we can also see the dependence of the field on the number of coils.

The circuit you should build includes a power supply, a variable resistor, a current meter and the Slinky, all connected in series. Use alligator clips to connect the other parts of the circuit to the Slinky. As noted above, you don't need to go from end-to-end, but can instead select a pre-determined number of coils.

However, we need to be a little careful in today's lab. It is fairly easy to dump so much current into a Slinky that it functions as a light bulb, or at a lower level, a heating element on an electric stove. Keep an eye on your currents, and don't power your circuit between measurements. It is easy to get burned. A good procedure would be to only touch the Slinky when you are changing a physical variable, and to use tape to hold down the circuit the rest of the time.

You lab instructor will cover the use of the magnetic field probe. Remember to zero the probe between measurements. Holding all other variables constant, use the probe while changing the current through the circuit. Make a graph and comment on your results. Do the same for any other variable you think is important. Use these graphs to generate a formula for the magnetic field of a Slinky.