High Tension (HT) leads consist of a single conductor covered in an insulation layer several times thicker than the conductor. This thick insulation layer - PVC, EDPM or silicone or a combination of them - is necessary to prevent arcing through the insulator to an earthed part of the engine.
Early HT leads used a copper conductor, sometimes tinned, made up of a number of separate strands. They work very well but do create radio interference which may be a problem. More modern types of HT leads use a conductor made of compressed carbon powder or stainless steel wire wound as a coil. Both of these later types of conductors have an inherent resistance and may be used in conjunction with plug caps and/or spark plugs each fitted with an internal resistor. This combined resistance acts to suppress any radio interference. Unfortunately it also reduces the voltage at the spark plug which means that, to achieve a spark at the plug, the magneto needs to generate a higher voltage in the first place. For this reason, magnetos should always use copper cored HT leads, unsuppressed plug caps and resistor-free spark plugs so that the magneto is not over stressed!