Page "Edison Disc Record" Paragraph 3

The grooves on an Edison Disc are smooth on the sides and have a variable depth.

Standard lateral discs will have a more constant depth, but the sides of the groove are scalloped.

As the Edison groove pitch ( or " TPI ", i. e. " threads per inch ") was 150, a much finer grooving than that on lateral discs, Edison's 10-inch discs played considerably longer than Victor's or Columbia's -- up to nearly five minutes per side.

The Edison Disc is also ¼-inch thick ( supposedly to prevent warping ), and was filled with wood flour, and later, china clay.