420 ALTERNATING CURRENTS 



The intensity in vertical planes passing through the axis of 

 the lamp varies considerably in different directions. This is 

 illustrated in Fig. 379, which shows the distribution in one 

 vertical plane of the light from an incandescent carbon-filament 

 lamp without a reflector. The intensity of the light in the- 

 vertically upward direction (180) is small owing to the pres- 

 ence of the base and socket and to the disposition of the fila- 

 ment. The intensity of the light in a vertically downward 

 direction is also small. 



These distribution curves are particularly useful in determin- 

 ing the suitability of a lamp for any particular purpose. The 

 distribution may be modified by shades, reflectors, etc. (See 

 Fig. 400, page 444.) As will be shown later, the area of these 

 distribution curves is not proportional to the total light flux 

 emitted by the lamp. 



186. Light Sources ; Incandescence ; Luminescence. Light 

 is emitted by sources under two conditions, incandescence and 

 luminescence. Incandescence is produced by heating a sub- 

 stance to a high temperature, as in the incandescent lamp, the 

 carbon arc, and the Welsbach gas mantle. The amount of light 

 emitted by a substance increases very rapidly with its tempera- 

 ature. In fact, the light emitted increases as the fourth power of 

 its absolute temperature. Hence, a small increase in the tem- 

 perature of an illuminant results in a very large increase in the 

 light which it emits. The light becomes more nearly white as 

 the temperature rises. This principle should be kept in mind 

 because it is the reason for the different efficiencies which are 

 obtained with different types of lamps. 



Light emitted by incandescent bodies is accompanied by high 

 temperature and a corresponding dissipation of considerable 

 energy as heat. On the other hand, a luminescent substance 

 may give out light at moderate temperatures. Examples of 

 luminescent light sources are the vacuum tube, the mercury arc, 

 the flaming and luminous arcs, all of which will be discussed later. 

 The firefly is an excellent example of a luminescent source. 



187. Carbon-filament Lamp. The requirements of an incan- 

 descent filament are that it shall be highly refractory, that is, 

 the filament shall be able to withstand high temperatures without 

 rapid deterioration, and the filament shall be mechanically strong 



