Stylus for capacitive videodisc

Claim 1

A pick-up stylus for tracking in a groove in a recording disk having a conductive surface and a dielectric coating over said conductive surface for cooperating with said recording disk to provide capacitive variations between said stylus and said conductive surface when relative motion is established between said stylus and said recording disk, said stylus comprising an elongated bar of alumina having a conductive coating thereon, said coating comprising a conductive layer and a cementing layer sandwiched between said conductive layer and the surface of said alumina bar, said cementing layer comprising an upper layer portion facing said conductive layer and a lower layer portion facing said alumina bar, said upper layer portion being an oxide of a material of said conductive layer and said lower layer portion being a solid solution of alumina and an oxide of a metal other than aluminum, the melting temperature of which is lower than the alumina, said solid solution being a member selected from the group consisting of Al 2 O 3 -SiO 2 , Al 2 O 3 -GeO 2 , Al 2 O 3 -SnO 2 , Al 2 O 3 -PbO, Al 2 O 3 -B 2 O 3 , Al 2 O 3 -Ga 2 O 3 , Al 2 O 3 -In 2 O 3 and combinations thereof, alumina-soda glass, alumina-lead glass, alumina-borosilicate glass, alumina-silica glass, alumina-pyrex glass and alumina-vycor glass, said conductive layer being a member selected from the group consisting of a metal, a metal nitride, a metal silicide, a metal boride, a metal carbide and combinations thereof.