ILLUMINATION AND PHOTOMETRY 433 



a gap. When current flow>. the starting: magnet. Fig. :>S9, brings 

 the lower electrode into contact with the upper electrode, striking 

 a sharp blow. This operation allows the current to flow through 

 the series magnet, thereby opening the circuit of the starting 

 magnet at the cut-out contacts, allowing the lower electrode to 

 fall, starting the arc. The lamp is then burning with the lower 

 electrode holder resting on its stop and with the series magnet 

 holding the cut-out contacts open. As the lower electn 

 consumed, the voltage across the arc rises, the shunt magnet 

 (which is connected in series with the starting magnet and start- 

 ing resistance across the arc) lifts its armature, and closi 

 cut-out contacts when the arc voltage has reached a predetermin- 

 ed value. The closing of these contacts puts the starting magnet 

 again in circuit, thereby causing the lower electrode to strike the 

 upper electrode a sharp blow, bringing about a shorter arc once 

 more. 



The copper anode, which is comparatively cool, is consumed 

 very slowly and needs to be replaced only at long intervals.' The 

 magnetite electrodes, however, need to be replaced at frequent 

 intervals. The 4.4-amp. lamp will burn from 160 to 200 hours 

 per trim. The 6.6-amp. lamp burns about 12") hours per trim. 

 Magnetite lamps have a high efficiency. The 4.4-amp. lamps 

 have an efficiency of approximately 0.70 watt per mean sph 

 candlepower. The 6.6-amp. lamps have an efficiency of 0.45 

 watt per mean spherical candlepower. The intense white light 

 of this type of lamp makes it very at t ract ive. particularly when 

 tin- lamp i< mounted on ornamental poles. In addition to gen- 

 eral street lighting, it is used to a considerable extent for "white 

 way" and boulevard lighting. (See Fig. 397, Page 443.) 



197. The Mercury-arc Lamp; the Moore Tube; the Nernst 

 Lamp. -The mercury-arc lamp CODH'M- of a long i ube containing 

 metallic mercury. 'The pressure in t he t ube is very low. ^ hen 

 an are i- formed in the tube the mercury is vapori/ed and 



'i-blue light. This light i- due almost wholly to the 

 luminescence of the mercury vapor, the temperature of the vapor 



only 250 to 300C. The lamp ov 



to the low temperature at which it is able its light. 



of the Unlit emitted come* from the blue'violet end of the 

 inn. th- little red. Thi> ab n e of red rays makes 



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