SCIENCE AND ELECTRIC LIGHTING 481 



Here was where research came to the rescue. A very brilliant series of 

 experiments showed the unexpected fact that if we coil up the filament 

 into a small space the movement of the gas past the filament is far less 

 effective in carrying away the heat than if the same filament is stretched out 

 straight. So we can use gas to suppress the evaporation and consequent 

 wastage of the tungsten, and still retain a much larger amount of heat in 

 the filament. 



Thus, here in this lamp is a similar filament wire coiled up into a fine 

 spiral. It is amply brilliant. There is plenty of gas in this bulb, but the 

 coiled filament is so much more retentive of heat in spite of the gas that we 

 can heat it right up — hotter for the same life than when it was straight. 

 When the lamp maker came to balance up the gain of light against the loss 

 of energy by the gas he found he was distinctly to the good on the balance. 



Let us illustrate this effect of the difference between a closely coiled wire 

 and an open coiled or straight wire. Here is a closely wound coil of thick 

 wire. As we pass electricity through it it rises to a red heat. The compact 

 form prevents the cooling air having very ready access to its surface and it 

 gets quite hot. But note what happens as we now straighten out the spiral. 

 There is the same electricity, everything is the same, except that now the 

 cooling air has far readier access, and the wire readily loses its heat to the 

 air which is streaming across it. 



Thus, we evolved the tightly coiled gas-filled filament lamp, and I hope 

 its advantage can now be seen. It held the field for twenty years, in fact 

 until a year ago. The coil of wire was extraordinarily fine, for it was 

 necessary to spiral a yard of the finest wire into a coil only i£ in. long. 



You will already have been asking why, if this coiling improves things 

 so much, we do not coil the coil, and make the hot element still more 

 compact and therefore preserve its heat still more. This has now been 

 done, but only after years of research — for the metallurgical and the 

 electrical problems involved have been very baffling. Here is such a coiled- 

 coil filament magnified. The larger proportion of the lamps made in this 

 country for domestic use now have coiled-coil filaments and they give up 

 to 20 per cent, more light for the electricity they use. 



Before we leave the filament lamp I think you ought to realise the skill 

 and craftsmanship which goes to the making of one of these filaments. 

 Consider the ordinary 40-watt lamp which we all use in our houses. The 

 metal tungsten out of which the filament is made is one of the toughest and 

 most refractory of metals. It is so hard that it has to be drawn hot through 

 diamond dies, which therefore have to be pierced with holes — perfectly 

 round holes — no more than the size of a fine human hair, for this is the 

 size of the finished wire. This wire diameter must be correct to within 

 a half of 1 per cent., i.e. five millionths of an inch. Picture, therefore, a 

 wire the size of a spider's web thread, accurate in diameter and beautifully 

 round and polished — and think of the craftsmanship of the girls who do 

 this drawing. As one of them described it to me, ' It is like threading a 

 wire you can't see through a hole that isn't there.' This minute thread 

 has then to be coiled with the greatest uniformity and equal precision, 

 and this coil again coiled. Out of the 3,775 turns in the spiral of one 

 filament not a single one must touch its neighbour, although there is only 

 a space less than the thickness of a cigarette paper between any two turns. 

 The whole diminutive structure has to carry electric current and stand 

 being heated to within 25 per cent, of its melting point, and remain so for 

 1,000 hours without distortion. Such filaments have to be made by the 

 thousand and such are your black looks if one occasionally fails prematurely, 

 that failures before 500 hours' burning have been reduced to less than 1 per 



