What it shows:
The magnetization of a ferromagnetic substance occurs in little jumps as the magnetic moments of small bunches of atoms, called domains, align themselves with the external field. We can actually "hear" the switching of these domains by amplifying the currents induced in a coil that surround the ferromagnetic material.
How it works:
We use two 10mH coils mounted back-to-back to cut out AC noise. The samples, listed in Fig.1 are in wire form, about 3-5cm in length and pushed through corks so they can sit snugly within the coils (Fig.2). The coils are connected to an amplifier which in turn feeds a speaker or hall PA. To align the domains, move one pole of a bar magnet slowly towards the wire from above; as the domains switch the induced pulse sounds like a series of clicks if you move the magnet in slowly, or as a hiss if you bring it in rapidly. Once the domains are aligned that particular pole will cause no further effect, but if you switch poles, you will of course now be flipping the domains in the other direction.
Table 1. Sample specifications
Figure 1. Setup of wire samples
Setting it up:
The coils we have are already fixed back-to-back with a Cannon jack to plug into the amp. We use a Shure Professional Microphone Mixer; go into any Mic input and out through Line Out into a speaker or the PA system. Set levels at mid-range and adjust to your satisfaction.
The thin samples give very distinct 'clicks' if you move in slowly; the thicker samples are better for the hissing of a whole load of domains (practice making train noises by flipping the magnet back and forth!) .The copper is there to show it as a ferromagnetic phenomenon.