Here are two examples of
aerial insulators. Porcelain being a very good insulator, it is still used
to this day. The white (and more common
one) on the left uses a hole and outer guide at right angles.
The brown insulator uses a wrap-around system
on two orthogonal axis. Glazed porcelain also has
the property that water will "bead" rather than coat, minimizing
the affect of rain causing a short. It is also easy to clean and
keep the chance of conductive contamination to a minimum.
Extract from Newnes
Encyclopaedia c. 1934.