Dimmer.

An adjustable choking coil used for regulating the intensity of electric incandescent lights. Some operate by the introduction and withdrawal of an iron core as described for the choking coil (see Coil, Choking), others by a damper of copper, often a copper ring surrounding the coil and which by moving on or off the coil changes the potential of the secondary circuit.

Dip of Magnetic Needle.

The inclination of the magnetic needle. (See Elements, Magnetic.)

Dipping.

(a) Acid or other cleaning processes applied by dipping metals in cleaning or pickling solutions before plating in the electroplater's bath.

(b) Plating by dipping applies to electroplating without a battery by simple immersion. Copper is deposited on iron from a solution of copper sulphate in this way.

Synonym--Simple Immersion.

Dipping Needle.

A magnet mounted in horizontal bearings at its centre of gravity. Placed in the magnetic meridian it takes the direction of the magnetic lines of force of the earth at that point. It is acted on by the vertical component of the earth's magnetism, as it has no freedom of horizontal movement. (See Magnetic Elements, and Compa.s.s, Inclination.)

Directing Magnet.

In a reflecting galvanometer the magnet used for controlling the magnetic needle by establishing a field. It is mounted on the spindle of the instrument above the coil and needle.

Synonym--Controlling Magnet.

186 STANDARD ELECTRICAL DICTIONARY.

Direction.

(a) The direction of an electric current is a.s.sumed to be from a positively charged electrode or terminal to a negatively charged one in the outer circuit. (See Current.)

(b) The direction of magnetic and electro-magnetic lines of force is a.s.sumed to be from north to south pole of a magnet in the outer circuit.

It is sometimes called the positive direction. Their general course is shown in the cuts diagrammatically. The circles indicate a compa.s.s used in tracing their course. The magnetic needle tends to place itself in the direction of or tangential to the lines of force pa.s.sing nearest it.

(c) The direction of electrostatic lines of force is a.s.sumed to be out of a positively charged and to a negatively charged surface.

Fig. 130. DIRECTION OF LINES OF FORCE OF A PERMANENT MAGNET.

Fig. 131, DIRECTION OF LINES OF FORCE OF AN ELECTRO-MAGNET.

187 STANDARD ELECTRICAL DICTIONARY.

Directive Power.

In magnetism the power of maintaining itself in the plane of the magnetic meridian, possessed by the magnetic needle.

Discharge, Brush.

The static discharge of electricity into or through the air may be of the brush or spark form. The brush indicates the escape of electricity in continuous flow; the spark indicates discontinuity. The conditions necessary to the production of one or the other refer to the nature of the conductor, and of other conductors in its vicinity and to the electro-motive force or potential difference; small alterations may transform one into the other. The brush resembles a luminous core whose apex touches the conductor. It is accompanied by a slight hissing noise.

Its luminosity is very feeble. The negative conductor gives a smaller brush than that of the positive conductor and discharges it more readily. When electricity issues from a conductor, remote from an oppositely excited one, it gives an absolutely silent discharge, showing at the point of escape a pale blue luminosity called electric glow, or if it escapes from points it shows a star-like centre of light. It can be seen in the dark by placing a point on the excited conductor of a static-electric machine.

Synonyms--Silent Discharge--Glow Discharge.

Discharge, Conductive.

A discharge of a static charge by conduction through a conductor.

Discharge, Convective.

The discharge of static electricity from an excited conductor through air or rarefied gas; it is also called the quiet or silent discharge.

The luminous effect in air or gas at atmospheric pressures takes the form of a little brush from a small positive electrode; the negative shows a star. The phenomena of Ga.s.siot's cascade, the philosopher's egg and Geissler tubes, all of which may be referred to, are instances of convective discharge.

Discharge, Dead Beat.

A discharge that is not oscillatory in character.

Discharge, Disruptive.

A discharge of a static charge through a dielectric. It involves mechanical perforation of the dielectric, and hence the mere mechanical strength of the latter has much to do with preventing it. A disruptive discharge is often oscillatory in character; this is always the case with the discharge of a Leyden jar.

188 STANDARD ELECTRICAL DICTIONARY.

Discharge, Duration of.

The problem of determining this factor has been attacked by various observers. Wheatstone with his revolving mirror found it to be 1/24000 second. Fedderson, by interposing resistance, prolonged it to 14/10000 and again to 138/10000 second. Lucas & Cazin made it from 26 to 47 millionths of a second. All these experiments were performed with Leyden jars.

Discharge, Impulsive.

A disruptive discharge produced between conductors by suddenly produced potential differences. The self-induction of the conductor plays an especially important part in discharges thus produced.

Discharge, Lateral.

(a) A lightning discharge, which sometimes takes place between a lightning rod and the building on which it is.

(b) In the discharge of a Leyden jar or condenser the discharge which takes the alternative path, q. v.

Discharge, Oscillatory.

The sudden or disruptive discharge of a static condenser, such as a Leyden jar, or of many other charged conductors, is oscillatory in character. The direction of the currents rapidly changes, so that the discharge is really an alternating current of excessively short total duration. The discharge sends electro-magnetic waves through the ether, which are exactly a.n.a.logous to those of light but of too long period to affect the eye.

Synonym--Surging Discharge.

[Transcriber's note: Marconi's transmission across the English channel occurs in 1897, five years after the publication of this book.]

Fig. 132. DISCHARGER.