NavList:

Seriously though, it’s a common feature of sextant certificates that companies tend to use the same basic certificates for decades with only minor updating.  Witness Hughes’ ladies with the bare ……s.  Although the certificate in question must be post-Litton, its quite possible that original drop of solder advice stems from towards the end of WW2 or even WWI when, because of the Allied blockade, there’s no saying which ersatz materials the Germans included in their torch batteries.  If the + cap to the carbon rod contained mainly brass (copper and zinc), the verdigris produced would be a mixture of Copper oxide, copper chloride, and copper carbonate depending upon how long the sextant or battery supplies spent at sea.  Although copper is an excellent conductor of electricity, oxides of copper are not.  That drop of solder (tin and lead) would prevent the build-up of verdigris.

This is one of the reasons why if you rewire your boat or add extra electrical equipment, you should pay extra for tinned-wire connecting cable.  Otherwise, salt laden atmosphere gets up the cut ends of plastic-coated wire and starts to affect the conductivity of the induvial strands.  It’s the same if your TV reception at home starts to fail.  The chances are that rain’s got into the connecting cable from the aerial to the TV to produce copper carbonate. 

The bare zinc on the - bottom of old-time dry batteries will also influence this cocktail, but that analysis a bit beyond my paygrade. DaveP