1907.] 071 Incandescent Illiiminants. 621 



supply off, and allowing a little gas to pass at the controlling tap, the 

 attachment to each burner turns off the burner and lights a little 

 pilot jet, which keeps ahght until the light is wanted again. On 

 turning on the main tap, the pilot jets light the various burners and 

 go out themselves. By this means burners can be fully Hghted up 

 by turning one tap at the door of the room. 



The electric incandescent light is undergoing a great change. 

 Carbon is being replaced by metal wires. It has been found possible 

 to make wires of high enough resistance of tungsten, osmium, tanta- 

 lum, and a few other metals and compounds. The osmium lamp was 

 the first of these, but there was difficulty in making it of high 

 enough resistance. The tantalum lamp is now in great demand. It 

 is made for lOO to 130 volts, and is much more efficient than the 

 carbon lamp. It will not last long on alternating currents, however. 

 The wires of a lamp that have been run for some time on a direct 

 current show a curious notched or crinkled appearance under the 

 microscope. But a wire that has been run on an alternating circuit 

 looks as if the metal had been melted into short cylinders with round 

 ends, and these cylinders had stuck together end to end without their 

 centres being in a line. Sometimes the little cylinders are nearly 

 separated, merely touching at a corner. This action is very extra- 

 ordinary, and has never been explained. In addition to this, when, a 

 lamp breaks down on an alternating circuit, the wire sometimes goes 

 at one point, and sometimes it breaks in several places, and tangles 

 itself up in an extraordinary way ; at other times it breaks up into 

 numerous little pieces, which will be found lying on the inside of the 

 globe. Some of the other lamps show a change under the action of 

 the current, but it is not so marked as in the case of tantalum. 



One of the most interesting of the new lamps is the Zircon. It 

 is said to be made of zirconium and tungsten, and lamps of this 

 material have been made for 200 volts, a matter of the greatest 

 importance from a distribution point of view. It is possible that the 

 conductor is really a zirconide of tungsten, and this opens up a new 

 series of compounds. A Zircon lamp for 100 volts has really six 

 separate loops of wire mounted in series inside a bulb. A recent 

 improvement is to provide an extremely light spring for each loop, so 

 as to keep it taut. The lamp can then be used in any position. 



Tungsten seems to be the favourite metal, as it gives a very high 

 efficiency. It is probable the lamp of the future will have an 

 efficiency of nearly a candle per watt, and this is promised by the use 

 of tungsten. At the same time it must be admitted that to make a 

 wire with a resistance of 500 ohms small enough to give 20 candles 

 with 20 watts is a triumph of inventive skill. 



[J. S.] 



[_The Lecture was illustrated hy numerous Experiments.'] 



