Maryanne! Stop doing that! Not only is it dangerous to apply ANY glass or ceramic material too an electric burner, Pyrex, Kimax and even borosilcate glass has a habit of weakening over time and it will sooner or later fall apart as attested to above. I had a Pyrex pitcher drop apart in a series of rings in dishwater.

Electric burners get much hotter than the dial indication, reason being the heat will be dispersed over a wider area. Glass holds heat well and does NOT desprse heat quickly. Melting can occur at points of direct contact. Also, even though Pyrex and like material is prestressed, such great variances between the coil and the mean temperature of the vessel will exceed its ability to expand uniformly, that is to say, it may explode like a glass grenade!