Separable Helices

   This pair of  Separable Helices  can be found in the 1912 catalogue of C.H. Stoelting of Chicago. The catalogue copy reads:

   "Primary and Secondary Coil, for use with galvanometer, wound with silk covered wire and supplied with movable rod of soft iron ... $5.50"

   The apparatus is in the Greenslade Collection.

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   I found the set of separable helices at the left at Westminster College in western Pennsylvania. They are by the L.E. Knott Apparatus Company of Boston, and are described in the 1916 Knott catalogue as:

   "Large Primary and Secondary Coil, for the study of current induction. This form, with its primary 6 inches by 1¼ inches and its secondary 2½  by 5 inches, must not be confused with the smaller types on the market. We have desired in this instrument to offer the teacher an instrument capable of efficient positive results. This can be secured in a satisfactory manner only by using a large number of windings on the secondary circuit. The wire on each circuit is mounted on separate hardwood spools, and the secondary is mounted on a neatly polished hardwood base. To insure greater rigidity, we have added two brass rods connecting the secondary to the base. The wire used is silk covered, and binding posts are neatly polished and lacquered. The soft iron core [missing] supplied with the instrument consists of a bundle of soft iron wires tightly moulded by bands to form a nearly solid rod. ... $7.50"