This section is from the book "Applied Science For Metal Workers", by William H. Dooley. Also available from Amazon: Applied Science For Metal Workers.
A device to resist or check the flow of current is commercially called a resistance box. It generally consists of an insulated wire, wound in a spool, the ends or terminals of which are fastened to large brass blocks. If the spools contain a large amount of silk, moisture tends to accumulate and cause inaccuracy. The plugs should be cleaned with coarse paper.
 
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