816 The Outline of Science 



exhaustion, a little inert gas which has no effect on the filament; 

 the lamp is then known as gas-filled. Adding the gas makes 

 the lamp more economical, since it gives considerably more light 

 per unit of current consumed, and this is why the gas-filled lamp, 

 made in strengths up to several thousand candle-power, has 

 largely replaced the arc lamp. In an arc lamp two carbon pen- 

 cils, connected with the circuit, are brought together point to 

 point. The tips become white-hot, and if they now be separated 

 slightly, atoms of incandescent carbon leap across the gap, from 

 one tip to the other, in a continuous and intensely luminous 

 stream, which is called an arc, because the path of its particles 

 is curved. An arc lamp includes apparatus which automati- 

 cally keeps the pencils the correct distance apart as they burn 

 away; allows them to fall together when the current is switched 

 off; and draws them apart to start the arc when current is 

 switched on. Though more troublesome than the incandescent 

 filament lamp, the arc can be much more powerful; and, as its 

 light proceeds from a very small area, the rays may be accurately 

 focussed by lenses and mirrors and projected in an intensely 

 luminous beam, such as is required for searchlights and cinema 

 projectors. Arc lights of up to 90,000,000 candle-power are 

 used in light-houses. There was, and perhaps still is, at a sta- 

 tion on the Jungfrau Railway a lamp that projected a beam 

 visible for sixty miles, which enabled a newspaper to be read in 

 the streets of Thun, thirty-five miles away. 



Though, where current is cheap, electric is the cheapest pos- 

 sible form of artificial lighting, besides being far the most con- 

 venient, from a scientific point of view it leaves much to be 

 desired. About nineteen-twentieths of the current that produces 

 it is dissipated as heat; and where it depends ultimately on coal, 

 only about one part in a hundred of the coal's energy is converted 

 into light. It has been said that if we could convert energy 

 into light as economically as the glow-worm, a single boy turn- 

 ing a handle could light a fair-sized town. There are evi- 



