The lower cork is then slowly
withdrawn, by twisting, until the core slowly rises.
[Illustration: Galvanoscope]
The instrument will then be adjusted ready for use. Connect the
binding posts to a single cell of battery--any kind will do, as a
slight current will answer. On completing
[Illustration: Interior View]
the circuit the core will descend; or put in a switch or push
button on one of the battery wires. If the button be concealed
where the operator can reach it, the core will obey his command to
rise or fall, according to his control of the current. This is a
mysterious looking instrument, the core being moved without
visible connection to any other part.
** Lubricating Sheet Metal [69]
To lubricate sheet metal mix 1 qt. whale oil, 1 lb. white lead, 1
pt. water and 3 oz. finest graphite. Apply with a brush before the
metal enters the dies.
** An Optical Top [69]
One of the latest optical delusions, and one not easy to explain,
is Benham's color top. Cut out the black and white disk shown in
the figure, and paste on a piece of stiff cardboard. Trim the
edges of the cardboard to match the shape of the disk, and make a
pinhole in the center.
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