This method relies upon the conductivity of the wood to transmit heat from the contact elements to the glue-line. The actual elements can be 26 gauge stainless steel or mild sheet steel plated against corrosion, and are heated by low-voltage high-amperage currents from a suitable stepdown transformer giving a range of tappings of low voltages suitable for normal work. The heat is generated by the resistance of the metal sheet to the passage of the current, and while stainless steel has more resistance and therefore more heat generated for a given power-supply, mild steel is cheaper and more readily obtainable. In practice, therefore, narrow elements up to 6 in (152 mm) wide used for edge banding, etc. are generally in mild steel, while wider elements are made of stainless steel, otherwise the amperage required to attain the degree of heat necessary to cure the glue would probably be in excess of the capacity of the transformer.
The metal sheets or 'elements', which can be either flat or curved to conform to whatever shape of jig is used, are laid in contact with the work, and it has been established in practice that various power ratings per square foot are necessary for differently shaped elements.
It follows, therefore, that as the amperage increases rapidly with the width of the element, some means must be adopted of reducing the amount of current required, and two standard methods are used. As the voltage requirements are low the length of the element can be increased at the expense of the width, either by (a) connecting a series of narrow strips of identical width with brass plates, nuts and bolts (325:1) so that the square element now becomes a continuous ribbon, or (b) cutting slots in a square sheet alternately from each side to form the continuous ribbon (325:2). If the latter method is adopted the ends of the cuts must be pronged as shown in 325:4 with each prong one-third of the width (W) and at an angle (A) of 90° to each other, or local hot spots will occur and the end of the strip will be much cooler. For all practical purposes method (a) is better, but whichever is used the strips must be identical in width, and cable connections, brass plates, bolts, etc. clean and firmly seated. Measurement of the effective temperature in any part of the continuous strip can be made with a thermo-couple; while paints and crayons which change colour when the temperature reaches certain values are obtainable.
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