Somewhat later halftones of photos were available, and these were
similarly bent into cylinders. The interstices between the metal points
of the halftones were filled with an insulating wax, and the whole
smoothed off until the bright metal points (of different size and
representing the different values of the picture) were exposed.
When this picture cylinder was rotated under a contact point, the
cylinder and the point being parts of an electric circuit, current
flowed in the circuit whenever the point touched the metal parts of the
picture, but no current flowed when the insulation passed under the
point.
Because the point does not jump up and down, but has a smooth surface to
ride on, greater speed and accuracy is possible with the filled-in
etching.