That black stuff is not crude oil.
It was branded in the 1970's as ferrofluid, consisting of an oil carrier (which isn't magnetic), and ferrous particles (which are). As such you wind up with a fluid which consists of iron particles suspended in oil, rather than dissolved in it. It was a clever pack of additives that maintained the surface tension of the oil carrier so that it behaved in the manner a truly magnetic liquid would behave.
It was fashionable to use it to cool high frequency drivers of PA speakers, and in some cases, certain domestic loudspeakers with very very high power handling requirements. I believe this practice is now obsolete with modern high power speakers using voice coils made from materials other than straight copper. By high power, I mean of the order of 1000 watts continuous RMS input. |