A cell in which two fluids are used, one generally as depolarizer surrounding the negative plate, the other as excitant surrounding the positive plate. A porous diaphragm or difference in specific gravities is used to keep the solutions separate and yet permit the essential electrolytic diffusion. Grove's Cell, Bunsen's Cell, and Daniell's Cell, all of which may be referred to, are of this type, as are many others.

Double Wedge.

A plug for use with a spring-jack. It has connection strips at its end and another pair a little distance back therefrom, so that it can make two loop connections at once.

Synonym--Double Plug.

Doubler.

A continuously acting electrophorous, q.v.; an early predecessor of the modern electric machines. It is now no longer used.

D. P.

Abbreviation for Potential Difference.

Drag.

The pull exercised by a magnetic field upon a conductor moving through it or upon the motion of an armature in it.

Dreh-strom. (German) Rotatory currents; a system of currents alternating in periodic succession of phases and producing a rotatory field. (See Field, Rotatory--Multiphase Currents.)

Drill Electric.

A drill for metals or rock worked by an electro-magnetic motor. For metals a rotary motion, for rocks a reciprocating or percussion action is imparted. It is used by shipbuilders for drilling holes in plates which are in place in ships, as its flexible conductors enable it to be placed anywhere. For rock-drilling a solenoid type of construction is adopted, producing rapid percussion.

192 STANDARD ELECTRICAL DICTIONARY.

Drip Loop.

A looping downward of wires entering a building, so that rain water, as it runs along the wire, will drip from the lowest part of the loop instead of following the wire into or against the side of the building.

Driving Horns.

Projections on the periphery of an armature of a dynamo for holding the winding in place and preventing its displacement. Various arrangements have been adopted. They are sometimes wedges or pins and are sometimes driven into s.p.a.ces left in the drum core. The toothed disc armature cores make up an armature in which the ridges formed by the teeth form practically driving horns.

Dronier's Salt.

A substance for solution for use in bichromate batteries. It is a mixture of one-third pota.s.sium bichromate and two-thirds pota.s.sium bisulphate. It is dissolved in water to make the exciting fluid.

Drop, Automatic.

A switch or circuit breaker, operating to close a circuit by dropping under the influence of gravity. It is held up by a latch, the circuit remaining open, until the latch is released by a current pa.s.sing through an electro-magnet. This attracting an armature lets the drop fall. As it falls it closes a local or second circuit, and thus may keep a bell ringing until it is replaced by hand. It is used in burglar alarms, its function being to keep a bell ringing even though the windows or door by which entrance was made is reclosed.

193 STANDARD ELECTRICAL DICTIONARY.

Fig. 133. THE MAGIC DRUM.

Drum, Electric.

A drum with a mechanism within for striking the head with a hammer or some equivalent method so as to be used as a piece of magical apparatus.

In the one shown in the cut a sort of telephone action is used to produce the sound, the electro-magnet D and armature being quite screened from observation through the hole. (See Fig. 133) A ring, C, shown in Fig. 133, with two terminals, the latter shown by the unshaded portions a a, and a suspending hook E, also with two terminals, and two suspending conductors A, B, carry the current to the magnet. A sudden opening or closing of the circuit produces a sound.

Dub's Laws.

1. The magnetism excited at any transverse section of a magnet is proportional to the square root of the distance between the given section and the end.

2. The free magnetism at any given transverse section of a magnet is proportional to the difference between the square root of half the length of the magnet and the square root of the distance between the given section and the nearest end.

Duct.

The tube or compartment in an electric subway for the reception of a cable. (See Conduit, Electric Subway.)

Dyad.

A chemical term; an element which in combination replaces two monovalent elements; one which has two bonds or is bivalent.

Dyeing, Electric.

The producing mordanting or other dyeing effects on goods in dyeing by the pa.s.sage of an electric current.

Dynamic Electricity.

Electricity of relatively low potential and large quant.i.ty; current electricity as distinguished from static electricity; electricity in motion.

194 STANDARD ELECTRICAL DICTIONARY.

Dynamo, Alternating Current.

A dynamo-electric machine for producing an alternating current; an alternator. They are cla.s.sified by S. P. Thompson into three cla.s.ses--I.

Those with stationary field-magnet and rotating armature. II. Those with rotating field magnet and stationary armature. III. Those with both field magnet part and armature part stationary, the amount of magnetic induction from the latter through the former being caused to vary or alternate in direction by the revolution of appropriate pieces of iron, called inductors. Another division rests on whether they give one simple alternating current, a two phase current, or whether they give multi phase currents. (See Current, Alternating--Currents, Multiphase.)

A great many kinds of alternators have been constructed. Only an outline of the general theory can be given here. They are generally multipolar, with north and south poles alternating around the field. The armature coils, equal in number in simple current machines, to the poles, are wound in opposite senses, so that the current shall be in one direction, though in opposite senses, in all of them at anyone time. As the armature rotates the coils are all approaching their poles at one time and a current in one sense is induced in every second coil, and one in the other sense in the other coils. They are all in continuous circuit with two open terminals, each connected to its own insulated connecting ring on the shaft. As the coils pa.s.s the poles and begin to recede from them the direction changes, and the current goes in the other direction until the next poles are reached and pa.s.sed. Thus there are as many changes of direction of current per rotation as there are coils in the armature or poles in the field.

Fig. 134. ALTERNATING CURRENT DYNAMO WITH SEPARATE EXCITER MOUNTED ON MAIN SHAFT.

195 STANDARD ELECTRICAL DICTIONARY.

The field-magnets whose windings may be in series are often excited by a separate direct current generation. Some are self-exciting, one or more of the armature coils being separated from the rest, and connected to a special commutator, which rectifies its current.

By properly s.p.a.cing the coils with respect to the poles of the field, and connecting each set of coils by itself to separate connecting rings, several currents can be taken from the same machine, which currents shall have a constant difference in phase. It would seem at first sight that the same result could be attained by using as many separate alternators as there were currents to be produced. But it would be almost impossible to preserve the exact relation of currents and current phase where each was produced by its own machine. The currents would overrun each other or would lag behind. In a single machine with separate sets of coils the relation is fixed and invariable.