An advantage attaching to the use of a compass needle is that
the magnet repels the wrong end of the needle.
[Illustration: FIG. 161.--General arrangement of electric wind recorder.]
The brush and segments must be protected from he weather by a cover, either
attached to the segment platform or to the tube on which the vane is
mounted.
The spaces between the segments must be filled in flush with some
non-conducting material, such as fibre, vulcanite, or sealing-wax; and be
very slightly wider than the end of the contact arm, so that two segments
may not be in circuit simultaneously. In certain positions of the vane no
contact will be made, but, as the vane is motionless only when there is no
wind or none to speak of, this is a small matter.
XXIX. A STRENGTH-TESTING MACHINE.
The penny-in-the-slot strength-testing machine is popular among men and
boys, presumably because many of them like to show other people what their
muscles are capable of, and the opportunity of proving it on a graduated
dial is therefore tempting, especially if there be a possibility of
recovering the penny by an unusually good performance.
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