When two metals in good electrical
contact are immersed in some liquid that is capable of attacking both,
only that metal will be attacked which is the more electro-positive, or
which (the same thing in other words) is the more readily attacked by the
liquid, evolving the more heat during its dissolution. As long as this
action is proceeding, as long, that is, as some of the more electro-
positive material is present, the less electro-positive material will not
suffer. All that has to be done, therefore, to protect the walls of an
acetylene-holder tank and the sides of its bell is to hang in the seal,
supported by a copper wire fastened to the tank walls by a trustworthy
electrical joint (soldering or riveting it), a plate or rod of some more
electro-positive metal, renewing that plate or rod before it is entirely
eaten away. [Footnote: Contact between the bell and the rod may be
established by means of a flexible metallic wire; or a separate rod might
be used for the bell itself.] If the iron is bare or coated with lead
(paint may be overlooked), the plate may be zinc; if the iron is
galvanised, _i.e._, coated with zinc, the plate may be aluminium or
an alloy of aluminium and zinc.
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