179 STANDARD ELECTRICAL DICTIONARY.

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 Amp?re'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; pota.s.sium bichromate, 184 parts; sulphuric acid, 428 parts.

Demagnetization.

Removal of magnetism from a paramagnetic substance. It is princ.i.p.ally 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If a magnetized watch is made to turn rapidly at the end of a twisted string and is gradually brought near to and withdrawn from the poles of a powerful dynamo it may be considerably improved.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 By the same action of the pole changer the plates of the battery are gradually withdrawn from the solution so as to gradually reduce the magnetic field to zero while constantly reversing its polarity.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.

180 STANDARD ELECTRICAL DICTIONARY.

Density, Electric Superficial.

The relative quant.i.ty of electricity residing as an electric charge upon a unit area of surface. It may be positive or negative.

Synonyms--Density of Charge--Surface Density.

Dental Mallet, Electric.

A dentist's instrument for hammering the fillings as inserted into teeth. It is a little hammer held in a suitable handle, and which is made to strike a rapid succession of blows by electro-magnetic motor mechanism.

Depolarization.

(a) The removal of permanent magnetism. (See Demagnetization.)

(b) The prevention of the polarization of a galvanic cell. It is effected in the Grove battery by the reduction of nitric acid; in the Bunsen, by the reduction of chromic acid; in the Smee battery, mechanically, by the platinum coated or rather platinized negative plate. Other examples will be found under the description of various cells and batteries. A fluid which depolarizes is termed a depolarizer or depolarizing fluid or solution. (See Electropoion Fluid.)

Deposit, Electrolytic.

The metal or other substance precipitated by the action of a battery or other current generator.

Derivation, Point of.

A point where a circuit branches or divides into two or more leads. The separate branches then receive derived or partial currents.

Desk Push.

A press or push b.u.t.ton, with small flush rim, for setting into the woodwork of a desk.

Detector.

A portable galvanometer, often of simple construction, used for rough or approximate work.

Detector, Lineman's.

A portable galvanometer with a high and a low resistance actuating coil, constructed for the use of linemen and telegraph constructors when in the field, and actually putting up, repairing or testing lines.

Deviation, Quadrantal.

Deviation of the compa.s.s in iron or steel ships due to the magnetization of horizontal beams by the earth's induction. The effect of this deviation disappears when the ship is in the plane of the electric meridian, or at right angles thereto; its name is taken from the fact that a swing of the ship through a quadrant brings the needle from zero deviation to a maximum and back to zero.

181 STANDARD ELECTRICAL DICTIONARY.

Deviation, Semicircular.

Deviation of the compa.s.s in iron or steel ships due to vertical induction. (See Induction, Vertical.) The effect of this induction disappears when the ship is in the electric meridian. Its name is derived from the fact that a swing of the ship through half the circle brings the needle from zero deviation to a maximum and back to zero.

Dextrotorsal. adj.

Wound in the direction or sense of a right-handed screw; the reverse of sinistrotorsal, q. v.

Fig. 128. DEXTROTORSAL HELIX.

Diacritical. adj.

(a) The number of ampere turns, q. v., required to bring an iron core to one half its magnetic saturation, q. v., is termed the diacritical number.

(b) The diacritical point of magnetic saturation is proposed by Sylva.n.u.s P. Thompson as a term for the coefficient of magnetic saturation which gives a magnet core one-half its maximum magnetization.

Diagnosis, Electro.

A medical diagnosis of a patient's condition based on the action of different parts of the body under electric excitement.