The strap is usually of iron, and the pulley of hard wood.
A recording drum or integrating apparatus may be arranged on the pulley frames.
It works in conjunction with the disk and scroll, the cones, or the expanding pulley, to impart an intermittingly variable speed to the bobbin (each layer of the bobbin has its own particular speed which is constant for the full traverse, but each change of direction of the builder is accompanied by a quick change of speed to the bobbin).
When the cones are pressed together or engaged, their friction causes the pulley to rotate along with the shaft; when they are disengaged, the pulley is free to stand still.
The pulley E is driven from an axle of the carriage.
