In the early 1800s galvanometers could be constructed with the fine gauges of silk-covered copper or silver wires produced for decorative purposes, but when Faraday was making his classic electrical experiments in 1831 he needed a sturdier gauge of copper wire. Bare copper wire was available in many diameters for mechanical applications, but coils for electromagnetic investigations had to be insulated with string and calico. It was soon realized that the cotton-covered springy iron wire then used to hold out the brims of ladies' bonnets showed how copper wire might be similarly wrapped to provide a flexible insulation. The simple manual machines used by the bonnet-wire makers were readily adapted and improved, and a six-head version was built by William Henley. This craftsman's vision of the growing importance of insulated copper wire was abundantly justified, and he built up a large - but poorly organised - empire in the wire and cable trade. Henley's original multiple-head wrapping machine has been located in the Science Museum, London, and the associated silk-covered copper wire subjected to physical, chemical, and electrical testing. For comparison, the electrical conductivity of the 'mechanical grade' copper wire used by Faraday has also been determined.
展开▼
