Alexander Graham Bell was lucky. The few wires in his primitive phone circuit were correctly connected when he called for his assistant's help. Watson heard him and, as they say, the rest is history. Cable harnesses in vehicles and appliances are much more complex then Bell's and must be thoroughly tested. For example, a car's wiring is divided into several separate harnesses for the instrument panel, chassis, and engine. The chassis harness alone may contain more than 400 circuits; aircraft harnesses include thousands of connections.
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