I was just looking at a 1911 book called Singer Instructions for Art Embroidery, scanned by the LIbrary of Congress (
http://archive.org/details/singerinstructio00sing). It has some beautiful color plates. It says "Place a small iron washer or a spool weight on the spool to retard the movement of the upper thread. A spool weight (No. 9537) can be obtained at any Singer shop." I wonder if that advice still applies.