The Nernst lamp is still another form of incandescent lamp, several types of which are shown in Figs. 19, 20, 21, and 22.

This lamp uses for the incandescent material certain oxides of the rare earths, the oxides being mixed in the form of a paste, then squirted through a die into a string which is subjected to a roasting process forming the filament or glower material of the lamp as represented by the lower white line in Fig. 23. The more recent glowers are made hollow instead of solid. The glowers are cut to the desired length and platinum terminals attached. The attachment of these terminals to the glowers is an important process in the manufacture of the lamp. The recent discovery of additional oxides has led to the construction of glowers which show a considerable gain in efficiency over those previously used. The glowers are heated to incandescence in open air, a vacuum not being required.

Fig. 19. Westinghouse Nernst Multiple  Glower Lamp.

Fig. 19. Westinghouse Nernst Multiple- Glower Lamp.

As the glower is a non-conductor when cold, some form of heater is necessary to bring it up to a temperature at which it will conduct. Two forms of heater have been used. One of them consists of a porcelain tube shown just above the glower, Fig. 23, about which a fine platinum wire is wound; the wire is in turn coated with a cement. Two or more of these tubes are mounted directly over the glower, or glowers, and serve as a reflector as well as a heater. The second form of heater consists of a slender rod of refractory material about which a platinum wire is wound, the wire again being covered with a cement. This rod is then formed into a spiral which surrounds the glower in the vertical glower type, or is formed into the wafer heater, Fig. 24, now universally employed in the Westinghouse Nernst lamp with horizontal glowers. The wafer heater is bent so that it can be mounted with several sections parallel to the glower or glowers.

Fig. 20. Sectional View of Multiple Glower Westinghouse Nernst Lamp.

Fig. 20. Sectional View of Multiple Glower Westinghouse Nernst Lamp.

The heating device is connected across the circuit when the lamp is first turned on, and it must be cut out of circuit after the glowers become conductors in order to save the energy consumed by the heater and to prolong the life of the heater. The automatic cut-out is operated by means of an electromagnet so arranged that current flows through this magnet as soon as the glower becomes a conductor, and contacts in the heater circuit are opened by this magnet. The contacts in the heater circuit are kept normally closed, usually by the force of gravity.

Fig. 21. Sectional Views of Single Glower Westinghouse Nernst Lamp.

Fig. 21. Sectional Views of Single-Glower Westinghouse Nernst Lamp.

The conductivity of the glower increases with the increase of temperature - the material has a negative temperature coefficient - hence if it were used on a constant potential circuit directly, the current and temperature would continue to rise until the glower was destroyed. To prevent the current from increasing beyond the desired value, a ballast resistance is used in series with the glower. As is well known, the resistance of iron wire increases quite rapidly with increase in temperature, and the resistance of a fine pure iron wire is so adjusted that the resistance of the combined circuit of the glower and the ballast becomes constant at the desired temperature of the glower. The iron wire must be protected from the air to prevent oxidization and too rapid temperature changes, and, for this reason, it is mounted in a glass bulb filled with hydrogen. Hydrogen has been selected for this purpose because it is an inert gas and conducts the heat from the ballast to the walls of the bulb better than other gases which might be used.

Fig. 22. Westinghouse Nernst Screw Burner.

Fig. 22. Westinghouse Nernst Screw Burner.

All of the parts enumerated, namely, glower, heater, cut-out, and ballast, are mounted in a suitable manner; the smaller lamps have but one glower and are arranged to fit in an incandescent lamp socket, while the larger types are constructed at present with four glowers and are arranged to be supported in special fixtures, or the same as small arc lamps. All parts are mechanically arranged so that renewals may be easily made when necessary and it is not possible to insert a part belonging to one type of lamp into a lamp of a different type.

Fig. 23. Westinghouse Nernst Screw Burner with Globe Removed, Showing Glower and Tubular Heater.

Fig. 23. Westinghouse Nernst Screw Burner with Globe Removed, Showing Glower and Tubular Heater.

Fig. 24. Wafer Heater and Mounting

Fig. 24. Wafer Heater and Mounting.

The advantages claimed for the Nernst lamp are: high efficiency; a good color of light; a good distribution of light without the use of reflector; a long life with low cost of maintenance; and a complete series of sizes of units, thus allowing its adaption to practically all classes of illumination.

The lamp is constructed for both direct- and alternating-current service and for 110 and 220 volts. When the alternating-current lamp is used on a 110-volt circuit a small transformer, commonly called a convertor coil, Fig. 25, is utilized to raise the voltage ;at the lamp terminals to about 220 volts.

Data on the Nernst lamp in its present form are given in Table VIII, and Figs. 26 and 27 show the form of distribution curves.

Fig. 25. Convertor Coil

Fig. 25. Convertor Coil.

Table VIII. General Data On The Nernst Lamp

Lamp

Rating in Watts

Voltage

Current in

Amperes

Max.

Candle-

Power

Mean

Hemispherical C. P.

Watts per M. H. S.

from

Test

c. p.

66

110

.6

74

50

1.38

88

220

.4

105

77 .

1 2

1-Glower

A. C.

or

D. C.

110

110

1.0

131

96.4

1.2

220

.5

132

110

1.2

156

1 14

1.2

220

.6

264

220

1.2

345

231

1.2

2-Glower

A. C.

or

D. C.

396

220

1.8

528

359

1.15

3-Glower

528

220

2.4

745

504

1.09

4-Glower