Haldat's Figures.
With a pole of a strong bar magnet, used like a pencil, imaginary
figures are drawn upon a hard steel plate, such as a saw-blade. The
pattern is gone over several times. By dusting iron filings on a sheet
of paper laid over the steel plate, while horizontal, very complicated
magnetic figures are produced.
Hall's Experiment.
A cross of thin metal, such as gold leaf, is secured upon a pane of
glass. To two opposite arms a battery is connected in circuit with them.
To the other two arms a galvanometer is connected in circuit. If the
cross is put into a field of force whose lines are perpendicular
thereto, the galvanometer will disclose a constant current. The current
is pushed, as it were, into the galvanometer circuit. Other metals have
been used with similar results. They must be thin or the experiment
fails. If the arm receiving the battery current is horizontal, and if it
flows from left to right, and if the lines of force go from downward
through the cross, the current in the galvanometer circuit will flow
from the observer through the other arms of the cross, if the cross is
of gold, silver, platinum or tin, and the reverse if of iron. The
experiment has indicated a possible way of reaching the velocity of
electricity in absolute measure.
Hall Effect.
The effect observed in Hall's experiment, q. v.
Hall Effect, Real.
A transverse electro-motive force in a conductor through which a current
is passing produced by a magnetic field.
Hall Effect, Spurious.
A spurious electro-motive force produced in a conductor, through which a
current is passing by changes in conductivity of the conductor brought
about by a magnetic field.
Hanger Board.
A board containing two terminals, a suspending hook, and a switch, so
that an arc lamp can be introduced into a circuit thereby, or can be
removed as desired.
Harmonic Receiver.
A receiver containing a vibrating reed, acted on by an electro-magnet.
Such a reed answers only to impulses tuned to its own pitch. If such are
received from the magnet it will vibrate. Impulses not in tune with it
will not affect it. (See Telegraph, Harmonic.)
Head Bath, Electric.
A fanciful name for an electro-medical treatment of the head. The
patient is insulated by an insulating stool or otherwise. His person is
connected with one terminal of an influence machine. An insulated
metallic circle, with points of metal projecting inward or downward, is
placed about the head. The circle is connected with the other pole of
the machine. On working it a silent or brush discharge with air
convection streams occurs between the patient's head and the circle of
points.