424 ALTERNATING CURRENTS 



gas coming in contact witli the filament becomes heated and 

 rises. If the lamp is in an inverted position, as it should be in 

 commercial use, (Fig. 381), this heated gas passes up into the 

 neck. It carries with it the tungsten vapor, which is thus depo- 

 sited in the neck of the lamp where it cuts off no appreciable 

 amount of light. 



This gas sweeping through the filament has one very unde- 

 sirable effect which is not present in a vacuum lamp. The gas 

 carries heat away from the filament very rapidly by convection. 

 This cooling of the filament tends to decrease the lamp effi- 

 ciency. To minimize this effect, the filament, instead of being a 

 straight wire, criss-crossed back and forth on supports as it is in 

 the Mazda-B lamp, is wound in the form of a very fine helix 

 with the turns very close together, as shown in Fig. 381(6). This 

 keeps the filament in very compact form and so reduces the 

 convection losses. 



Below is given a table of efficiencies for gas-filled lamps. It 

 will be noted that the larger sizes have the higher efficiencies, 

 due to their having larger filaments and hence less proportionate 

 convection losses. 



RATING GAS-FILLED MULTIPLE LAMPS 



WATTS PER LUMENS 



WATTS M.H.CP. M.H.CP. PER WATT 



75 88 0.85 11.5 



100 120 0.83 12.6 



200 267 0.75 14.0 



500 714 0.70 16.1 



750 1,154 0.65 17.1 



1,000 1,667 0.60 18.0 



By using a bulb having a blue tint, the gas-filled lamp gives 

 a light closely resembling daylight. 



191. Effect of Voltage Variation on Incandescent Lamps. 

 As the voltage on commercial lighting systems will vary more 

 or less from time to time, it is important to understand the effect 

 of this voltage variation on the operation of incandescent lamps. 

 An increased voltage results in a higher operating temperature of 

 the filament and hence a higher efficiency. This is accompanied 

 however by a decreased life of the lamp. 



