1,145,265. Making transformer cores; making coils; welding by pressure. P. EISLER. 3 June, 1966 [3 June, 1965], No. 23641/65. Headings B3A, B3E and B3R. [Also in Division H1] A helicoid is made from continuous metal strip, e.g. copper or aluminium, by cutting and folding the strip, or by cutting and assembling two strips together and then folding or by cutting and assembling the cut-out parts of the strip together. In one embodiment of cutting and folding the strip to form the helicoid a metal strip (200), Fig. 1 (not shown), has openings cut in it and alternate longitudinal arms cut transversely and abutted with the adjacent longitudinal arm. The transverse arms 201, 203 are folded medially to form a double thickness helicoid. In a modification to avoid waste the openings are formed by folding back flaps (216), (217), Fig. 4 (not shown), and folding the transverse arms about three fold lines (221) to form a four thickness helicoid. The individual patterns may be relatively staggered and the edges 242 of the helicoid splayed out to facilitate cooling. The turns may be insulated by paper inserts (a), (b), Figs. 13-15 (not shown), after assembly or by using a laminated strip initially. The inserts may be oblong or L-shaped and may be laminated with metal foil. The metal foil can either be insulated from the helicoid by the paper to transfer heat to the edges of the helicoid or in contact with the helicoid to increase the current cross-section. The widths of the spacers may be varied, Fig. 18 (not shown). The insulation can be ribbed (246), Fig. 17 (not shown), to strengthen the helicoid. In an alternate embodiment the coils are made from metal strip 251, Fig. 19, which is cut into zig-zags 253. Each zig-zag strip is punched with complementary patterns 256, 258 which co-operate when a pair of strips are superimposed after being phase shifted. The double thickness strips are folded about lines 254, 255 to form a helicoid of uniform thickness. The strips may be laminated with paper insulation before cutting or the paper (276), Fig. 22 (not shown), pushed into the cut strip by knives 274 while the strip is clamped in the strip cutter. The knives 274 are pivoted on a plate 275 which oscillates to lay the paper over the metal strip, Fig. 23. In a modification of this embodiment the zig-zag strips have side flaps, Fig. 24 (not shown), which are shaped so that the helicoid is double thickness when they are folded inwards. To prevent waste of material the side strips (294), (295), Fig. 27 (not shown), may be ultrasonically welded together and used to form further zig-zags. In a further embodiment the helicoid is made of either rectangular or L-shaped stampings from a laminated strip of paper and metal. The pieces are superimposed with the metal faces together and opposed pieces are displaced so that the joints are staggered. Additional metal or paper layers may be placed between the layers. The transformer core is made from plain strip pieces 301, 302 of silicon steel by cutting across the grain. The steel may be insulated with paper on one side and each piece is arranged with the paper to the same side. The pieces are clamped between plates 306 by bolts 305 against which a wedge 307 is driven to move the pieces together, Fig. 34. The plates have flanges to clamp the coils. Packings strips 309 serve to distribute the load applied by bolts 308. The coils may have splayed ends in which case they are enclosed in a shroud made of two parts (312), Fig. 36 (not shown). This construction of transformer may be immersed in a liquid. A metal box 317 with end containers 318 can be fitted into the transformer window and filled with water or other transformer coolant. Leaf springs 319 engage the joints between the box and the containers to press the flat sides of the box against the windings within the window. Alternately the sides are secured to the windings with temperature resistant cement. In all the embodiments the coils may have rounded corners or be oval or circular.