* Fixed electrical contact for jack, key, relay, switch, etc.
Moving Contact, Adjustable or sliding contact for resistor, inductor, etc.
(in English) (1993)  315-1975 - IEEE Standard American National Standard Canadian Standard Graphic Symbols for Electrical and Electronics Diagrams (Including Reference Designation Letters), IEEE  DOI:  10.1109/IEEESTD.1993.93397. ISBN:  0738109479.  Page 52 for the first and 53 for the second.
Michel Bakni