This guide is nearly 90 years old, but not a great deal has changed since then which is relevant to joining wires by soldering.    

The tips on removing insulation don’t apply to modern indoor PCV cables, but the techniques for joining wires so that the mechanical strength is provided by the joint itself, not the solder (which should only be to ensure good electrical contact) are still of value.

19/03/2013 ban-all-sheds