8 SCIENCE PROGRESS 



as most people change their lamps when they begin to give too 

 poor a light. The useful life thus determined is generally 

 considerably longer, often as much as double, the economical 

 life but fortunately the loss in economy is not very great if the 

 lamp is kept running until the end of its useful life : the actual 

 life on the other hand is often very much longer than the useful 

 life and it never pays to run lamps long after their candle power 

 is noticeably small. In the table given above the figures refer 

 to useful life. 



Sufficient time has not yet elapsed for the general effect of 

 the introduction of these high-efficiency lamps on the electrical 

 industry to be apparent. At first, naturally, the central stations 

 suffered ; the new lamps were substituted for old ones in 

 installations already in existence and there was a fall in the 

 consumption of energy and consequently of revenue. But the 

 process of recovery is now begun, as the new lamps make 

 the competition of electric lighting with incandescent gas 

 lighting possible and a steady increase in its use is to be 

 expected confidently. It will doubtless be some time before 

 the cheapness of electric lighting with tungsten lamps is 

 generally realised but this once attained the progress should 

 be rapid. As with all inventions which cheapen a product to 

 the consumer, the ultimate effect on the industry cannot be 

 other than beneficial ; the cheapening in this instance being 

 very marked — to about one-third of the old cost — the benefit 

 should be correspondingly great. 



Parallel with the development of the incandescent electric 

 lamp, the small unit of light suitable for general interior lighting, 

 there has been a no less noteworthy development in the arc lamp, 

 the large unit suitable for exterior lighting and the lighting of 

 large interiors. The arc is nearly a century old and arc lamps 

 have been a commercial means of artificial lighting during a large 

 proportion of that time but until about five years ago only the 

 ordinary open or enclosed type of lamp was available. The 

 open arc is an arc burning between two carbon rods in the free 

 air, which is enclosed within a globe only to protect it from 

 wind and weather. The arc itself is very short (about 2 mm.) 

 and emits only an inappreciable percentage of the light given by 

 the lamp, this being mainly derived from the white-hot ends of 

 the carbons. In a direct current lamp, the positive carbon 

 burns to a blunt point which is slightly hollowed out, forming 



