482 EVENING DISCOURSES 



cent, in the best makes. People think lamps are expensive ! If the 

 filaments were ten times larger so that they could be seen I believe people 

 would think that they were worth more — notwithstanding the fact that 

 they would be a hundred times easier to make. But the lack of gratitude 

 in my fellows is not my theme this evening — so we will pass from the 

 filament lamps. 



Electric Discharge Lamps. 



We saw just now that the enhanced efficiency of this vapour lamp was 

 due to the absence of unwanted radiation. 



To show that the efficiency is actually much higher we have these two 

 lamps, a vapour lamp and a filament lamp, both using the same amount of 

 electricity. I think it is obvious from inspection that the vapour lamp 

 contributes much more to the total light than does the filament lamp. 

 Actually, in the comparison before us the amount is about three times as 

 great. 



Now all vapours and gases are not so generous as mercury in the amount 

 of light they give, so we have to choose carefully between them. 



There is a somewhat subtle fact here which we have to remember. Your 

 eyes and mine are not equally sensitive to all colours. Here is a figure which 

 shows by the height at different colours how much more readily our eyes 

 can see with a given amount of radiation in one colour, say yellow-green, 

 than with the same amount of radiation with blue or red colour. 



If we had two electric lamps giving out the same amount of light and 

 both using a pennyworth of electricity a day, but one giving only blue light 

 and the other only green light, the second one would appear to us to give 

 five times more light than the other, because to our eyes, as we see from 

 the diagram, the green light is more useful to see with ; in other words, it 

 seems brighter. 



Here are the colours given out by the gas argon. You see the principal 

 colours are in the red and violet end of the spectrum. There is also a little 

 blue, but very little orange or green. Here is an actual lamp with argon 

 gas in it. The colour is attractive, but you see it has not those colour 

 elements to which the eye is more sensitive and the lamp in consequence is 

 inefficient and on this account will hardly do for utility purposes. 



Here, on the other hand, is an electric discharge lamp which, in addition 

 to argon, contains some solid sodium. On first switching current on to 

 this lamp the colour obtained is very similar to that of the argon tube we 

 have just seen. If, however, we vaporise the sodium by heating it, the 

 brilliant yellow light characteristic of sodium is obtained. The colour is 

 far from attractive for general lighting but it is efficient, first because its 

 colour — yellow — is one to which our eyes are fairly sensitive, and secondly 

 because there is so little other radiation of a non-visible nature. 



Here are a number of different gases through which electricity is passing. 

 You see the large variety of colour which is possible, and can appreciate 

 how it is that we have such cheerful colour effects in our city streets at night. 

 For purely decorative work it is of course attractive to have as many colours 

 as possible to choose from. 



Some of these tubes are fairly efficient, especially when considered as 

 creators of coloured light. Neon gas, for example, with its rich orange-red 

 light is about three times as efficient as a tungsten filament lamp in a red 

 glass bulb and giving light of about the same colour. 



Each gas or vapour much prefers to operate alone. The presence of 

 two gases or vapours in the same tube usually results in the suppression of 

 the light from one of them. 



