Strap-copper coils with paper or other insulating material between turns merely require a strongtension and some hammering with a rawhide mallet or a piece of wood interposed between the coil and the hammer in order to cause the insulated strap or ribon of copper to lie properly in place. The insulation should be suficiently strong, mechanically, to withstand such treatment. In one form of winding machine, a roll of strap copper and a roll of strip paper, about double the width of the strap copper, are fed through a folding device after they leave their respective rolls in such a manner that the paper is folded longitudinally around the copper as the coil is being wound.
In an ingenious automatic winding procss, even the finer sizes of round magnet wire are alternately and spirally wound outward and inward to form a coil consisting of spiral disks which can be separated from one another at any point in the coil for repair or for other purposes, and then can be reassembled, the coil normally consisting of a single length of magnet wire. By this process, a very long coil may be constructed and then cut apart with a dull knife into desired lengths. All splices are made on the outside of the coil. In case of a defect, the defective portion may be removed as above described, and a new portion added to the sound portion.
An example of the spirally wound disk coil . The disks are 8o arranged and connected that the current flows in the same direction in each coil around the core hole. By this arrangement, the total voltage is across the length of the coil instead of aceross the coil-wall thickness, with respect to the entire coil.
The secondaries of large induction coils and high-tension transformer coils are constructed in a somnewhat similar manner, only each disk or pie, in the induction coil, consists of a coil of small coil length with relatively few turns per layer. The insulation between the primary and secondary must be suficient to withstand the entire voltage of the coil.