This device, as shown in the accompanying illustration, is a glass
cylinder fixed on an ebonite base, and closed at the top by an ebonite
cap. A solid brass rod runs from top to bottom, and near the bottom,
and at right angles to it, is fixed a smaller adjustable rod,
terminating in a flat head. Opposite to this flat disk there is a
brass strip secured to the ebonite cap. From the top of this brass
strip hangs a gold or aluminum foil. The foil and strip are placed to
earth, and the solid brass rod is connected to the circuit to be
protected. Should the difference of potential between the foil and the
terminal opposite to it attain more than a certain amount,
electrostatic attraction will cause the foil to touch the disk and
place the circuit to earth. The apparatus, which is a modification of
the Cardew earthing device, is constructed by Messrs. Drake & Gorham,
of Victoria Street.--_The Electrician_.
[Illustration.]
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