This is one of the easiest traps to build
and is usually successful.
[Illustration: Trap]
** Homemade Grenet Battery [83]
Procure an ordinary carbon-zinc, sal-ammoniac battery and remove
the zinc rod. If the battery has been used before, it is better to
soak the carbon cylinder for a few hours to remove any remaining
crystals of sal-ammoniac from its pores.
The truncated, conical zinc required is known as a fuller's zinc
and can be bought at any electrical supply dealer's, or, it may be
cast in a sand mold from scrap zinc or the worn-out zinc rods from
sal-ammoniac batteries. It should be cast on the end of a piece of
No. 14 copper wire. Amalgamation is not necessary for the zinc one
buys, but if one casts his own zinc, it is necessary to amalgamate
it or coat it with mercury. This may be done as follows:
Dip a piece of rag in a diluted solution of sulphuric acid (water
16 parts, acid 1 part); rub the zinc well, at the same time
allowing a few drops of mercury to fall on a spot attacked by the
acid. The mercury will adhere, and if the rubbing is continued so
as to spread the mercury, it will cover the entire surface of the
zinc, giving it a bright, silvery appearance.
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