On Monday, January 14, 2019 at 7:24:57 PM UTC-8, wrote:
I remember a lot of old transformers, particularly the power
transformers on the old TV sets, were coated with tar.

There are mechanical forces between wires carrying current, and in addition
to making 'hum' that puts some metal fatigue into the mix. For
reliability, especially for switchmode, vacuum impregnation or some other
secure-the-wires provision is common in power inductors. At low
frequencies, you can get away with oil and paper. Or thick oil (asphalt, tar...).