(koil) [colligere, to gather together]. A spiral formed by winding. C. -gland. See Sweat-gland. C, Induction-, rolls of wire used to produce an electric current by induc- tion. C, Letter's. See Letter's Tubes. C, Primary, the inner coil of an induction ap- paratus. C, Resistance-, a coil of wire of known electric resistance, used for estimat- ing resistance. C, Secondary, the* outer coil of an induction apparatus.