In casting zincs for batteries a separate crucible, used only for
zinc, is very desirable, as the presence of a very small amount of
lead or other impurity will cause the batteries to polarize. A
very good way to make the binding posts is to remove the binding
posts from worn-out dry batteries and place them in the molds in
such a way that the melted zinc will flow around them.
The time required for a casting to solidify varies with the size
and shape of the casting, but unless the pattern is a very large
one about five minutes will be ample time for it to set. The
casting is then dumped out of the mold and the sand brushed off.
The gate can be removed with either a cold chisel or a hacksaw,
and the casting is then ready for finishing.
** Battery Switch [99]
In cases where batteries are used in series and it is desirable to
change the strength and direction of the current frequently, the
following device will be found most convenient. In my own case I
used four batteries, but any reasonable number may be used.
Referring to the figure, it will be seen that by moving the switch
A toward the left the current can be reduced from four batteries
to none, and then by moving the switch B toward the right the
current can be turned on in the opposite
[Illustration: Battey Switch]
direction to the desired strength.
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