The building up of an armature core or other thing out of plates. The cores of dynamo armatures or of alternating current converters are often laminated. Thus a drum armature core may consist of a quant.i.ty of thin iron discs, strung upon a rod and rigidly secured, either with or without paper insulation between the discs. If no paper is used the film of oxide on the iron is relied on for insulation. The object of lamination is to break up the electrical continuity of the core, so as to avoid Foucault currents. (See Currents, Foucault.) The laminations should be at right angles to the direction of the Foucault currents which would be produced, or in most cases should be at right angles to the active parts of the wire windings.

319 STANDARD ELECTRICAL DICTIONARY.

Lamination of Armature Conductors.

These are sometimes laminated to prevent the formation of eddy currents.

The lamination should be radial, and the strips composing it should be insulated from each other by superficial oxidation, oiling or enamelling, and should be united only at their ends.

Fig. 210. PILSEN ARC LAMP.

Lamp, Arc.

A lamp in which the light is produced by a voltaic arc. Carbon electrodes are almost universally employed. Special mechanism, operating partly by spring or gravity and partly by electricity, is employed to regulate the distance apart of the carbons, to let them touch when no current pa.s.ses, and to separate them when current is first turned on.

The most varied constructions have been employed, examples of which will be found in their places. Lamps may in general be divided into cla.s.ses as follows, according to their regulating mechanism and other features:

(a) Single light regulators or monophotes. Lamps through whose regulating mechanism the whole current pa.s.ses. These are only adapted to work singly; if several are placed in series on the same circuit, the action of one regulator interferes with that of the next one.

(b) Multiple light regulators or polyphotes. In these the regulating mechanism and the carbons with their arc are in parallel; the regulating device may be a single magnet or solenoid const.i.tuting a derived or shunt-circuit lamp, or it may include two magnets working differentially against or in opposition to each other const.i.tuting a differential lamp.

320 STANDARD ELECTRICAL DICTIONARY.

(c) Lamps with fixed parallel carbons termed candles (q. v., of various types).

(d) Lamps without regulating mechanism. These include lamps with converging carbons, whose object was to dispense with the regulating mechanism, but which in some cases have about as much regulating mechanism as any of the ordinary arc lamps.

Lamp, Contact.

A lamp depending for its action on loose contact between two carbon electrodes. At the contact a species of incandescence with incipient arcs is produced. One of the electrodes is usually flat or nearly so, and the other one of pencil shape rests upon it.

Lamp, Differential Arc.

An arc lamp, the regulation of the distance between whose carbons depends on the differential action of two separate electrical coils. The diagram ill.u.s.trates the principle. The two carbons are seen in black; the upper one is movable, The current arrives at A. It divides, and the greater part goes through the low resistance coil M to a contact roller r, and thence by the frame to the upper carbon, and through the arc and lower carbon to B, where it leaves the lamp. A smaller portion of the current goes through the coil M1 of higher resistance and leaves the lamp also at B. A double conical iron core is seen, to which the upper carbon holder is attached. This is attracted in opposite directions by the two coils. If the arc grows too long its resistance increases and the coil M1 receiving more current draws it down and thus shortens the arc. If the arc grows too short, its resistance falls, and the coil M receives more current and draws the core upwards, thus lengthening the arc. This differential action of the two cores gives the lamp its name.

R is a pulley over which a cord pa.s.ses, one end attached to the core and the other to a counterpoise weight, W.

Fig. 211. DIAGRAM OF THE PILSEN DIFFERENTIAL ARC LAMP.

321 STANDARD ELECTRICAL DICTIONARY.

Lamp, Holophote.

A lamp designed for use alone upon its own circuit. These have the regulating mechanism in series with the carbon and arc, so that the whole current goes through both. (See Lamp, Arc.)

Synonym--Monophote Lamp.

Lamp-hour.

A unit of commercial supply of electric energy; the volt-coulombs required to maintain an electric lamp for one hour. A sixteen-candle power incandescent lamp is practically the lamp alluded to, and requires about half an ampere current at 110 volts, making a lamp-hour equal to about 198,000 volt-coulombs.

[Transcriber's note: 0.55 KW hours.]

Lamp, Incandescent.

An electric lamp in which the light is produced by heating to whiteness a refractory conductor by the pa.s.sage of a current of electricity. It is distinguished from an arc lamp (which etymologically is also an incandescent lamp) by the absence of any break in the continuity of its refractory conductor. Many different forms and methods of construction have been tried, but now all have settled into approximately the same type.

The incandescent lamp consists of a small gla.s.s bulb, called the lamp-chamber, which is exhausted of air and hermetically sealed. It contains a filament of carbon, bent into a loop of more or less simple shape. This shape prevents any tensile strain upon the loop and also approximates to the outline of a regular flame.

Fig. 212. INCANDESCENT ELECTRIC LAMP.

322 STANDARD ELECTRICAL DICTIONARY.

The loop is attached at its ends to two short pieces of platinum wire, which pa.s.s through the gla.s.s of the bulb and around which the gla.s.s is fused. As platinum has almost exactly the same coefficient of heat-expansion as gla.s.s, the wires do not cause the gla.s.s to crack.

The process of manufacture includes the preparation of the filament.

This is made from paper, silk, bamboo fibre, tamidine, q. v., or other material. After shaping into the form of the filament the material is carbonized at a high heat, while embedded in charcoal, or otherwise protected from the air. The flashing process (see Flashing of incandescent Lamp Carbons) may also be applied. The attachment to the platinum wires is effected by a minute clamp or by electric soldering.

The loop is inserted and secured within the open globe, which the gla.s.s blower nearly closes, leaving one opening for exhaustion.

The air is pumped out, perhaps first by a piston pump, but always at the end by a mercurial air pump. (See Pump, Geissler--and others.) As the exhaustion becomes high a current is pa.s.sed through the carbons heating them eventually to white heat so as to expel occluded gas. The occluded gases are exhausted by the pump and the lamp is sealed by melting the gla.s.s with a blowpipe or blast-lamp flame. For the exhaustion several lamps are usually fastened together by branching gla.s.s tubes, and are sealed off one by one.

The incandescent lamps require about 3.5 watts to the candle power, or give about 12 sixteen-candle lamps to the horse power expended on them.

Generally incandescent lamps are run in parallel or on multiple arc circuits. All that is necessary in such distribution systems is to maintain a proper potential difference between the two leads across which the lamps are connected. In the manufacture of lamps they are brought to an even resistance and the proper voltage at which they should be run is often marked upon them. This may be fifty volts and upward. One hundred and ten volts is a very usual figure. As current one ampere for a fifty-volt, or about one-half an ampere for a one hundred and ten volt lamp is employed.

Lamp, Incandescent, Three Filament.

A three filament lamp is used for three phase currents. It has three filaments whose inner ends are connected, and each of which has one leading-in wire. The three wires are connected to the three wires of the circuit. Each filament receives a current varying in intensity, so that there is always one filament pa.s.sing a current equal to the sum of the currents in the other two filaments.

Lamp, Lighthouse.

A special type of arc light. It is adapted for use in a lighthouse dioptric lantern, and hence its arc has to be maintained in the same position, in the focus of the lenses. The lamps are so constructed as to feed both carbons instead of only one, thereby securing the above object.

323 STANDARD ELECTRICAL DICTIONARY.

Lamp, Pilot.

A lamp connected to a dynamo, and used by its degree of illumination to show when the dynamo on starting becomes excited, or builds itself up.

Lamp, Polyphote.