Degeneration, Reaction of.
The diminished sensibility to electro-therapeutic treatment exhibited by
the human system with continuance of the treatment in question. The
general lines of variation are stated in works on the subject.
Deka.
Prefix originally used in the metric system to signify multiplying by
ten, as dekameter, ten meters, dekagram, ten grams; now extended to many
scientific terms.
De la Rive's Floating Battery.
A small galvanic couple, immersed in a little floating cell and
connected through a coil of wire immediately above them. When the
exciting battery solution is placed in the cell the whole, as it floats
in a larger vessel, turns until the coil lies at right angles to the
magnetic needle. Sometimes the two plates are thrust through a cork and
floated thus in a vessel of dilute sulphuric acid.
A magnet acts to attract or repel the coil in obedience to Ampére's
Theory, (See Magnetism, Ampere's Theory of.)
Delaurier's Solution.
A solution for batteries of the Bunsen and Grenet type. It is of the
following composition:
Water, 2,000 parts;
potassium bichromate, 184 parts;
sulphuric acid, 428 parts.
Demagnetization.
Removal of magnetism from a paramagnetic substance. It is principally
used for watches which have become magnetized by exposure to the
magnetic field surrounding dynamos or motors.
The general principles of most methods are to rotate the object, as a
watch, in a strong field, and while it is rotating to gradually remove
it from the field, or to gradually reduce the intensity of the field
itself to zero. A conical coil of wire within which the field is
produced in which the watch is placed is sometimes used, the idea being
that the field within such a coil is strongest at its base. Such a coil
supplied by an alternating current is found effectual (J. J. Wright).
If a magnetized watch is made to turn rapidly at the end of a twisted
string and is gradually brought near to and withdrawn from the poles of
a powerful dynamo it may be considerably improved.
A hollow coil of wire connected with a pole changer and dip-battery has
been used. The battery creates a strong field within the coil. The watch
is placed there and the pole changer is worked so as to reverse the
polarity of the field very frequently. By the same action of the pole
changer the plates of the battery are gradually withdrawn from the
solution so as to gradually reduce the magnetic field to zero while
constantly reversing its polarity. (G. M. Hopkins.)
Steel may be demagnetized by jarring when held out of the magnetic
meridian, or by heating to redness.