The current required is
very small, as the core is so nearly balanced that the least
attraction will cause it to sink.
The glass tube may be a test tube, as shown in Fig. 2, or an empty
developer tube. If one has neither a test tube nor developer tube,
an empty pill bottle may be used. The washers at the ends of the
coil can be made of fiber, hard rubber, or wood; or can be taken
from an old magnet. The base may be made of wood or any other
insulating material and should have four short legs on the bottom.
Make the coil of single-covered wire about No. 18 and connect ends
to binding posts as shown in Fig. 2.
The core is made by pushing a small nail through a piece of cork.
It should be made so that it will rise slowly when placed under
water. Some filing may be necessary to get the weight just right,
but it should be remembered that the buoyancy of the core can be
adjusted after the parts are assembled, by pressing the cork in
the bottom of the test tube. This causes compression in the water
so that some is forced into the upper cork, reducing its
displacement and causing it to sink.
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