December 18, 1902] 



NA TURE 



163 



same colour as the crater. If it has an enormous specific heat, 

 it ought to raise the other pole to crater temperature where it 

 condenses. If it is a light gas, a large portion of its specific 

 heat of vaporisation may go to external work. Most of the 

 upper carbon is burnt away by external air ; if a pencil to match 

 the crater is volatilised, it does not account for much power. If 

 the vapour is very light, there must be large volumes from the 

 upper carbon. Then what conducts ? Carbon vapour alone, or 

 mixed with a little monoxide or nitrogen, is a very good con- 

 ductor at these temperatures. Does that go to show that carbon 

 vapour dissociates like iodine or chlorine, &c. ? The whole 

 question of the physics of the arc deserves far more careful study 

 than it has yet received, but the work is surrounded with diffi- 

 culties and is really a branch of the theory of the passage of 

 electricity through gases, a matter of the greatest scientific 

 importance, somewhat out of our way as practical electrical 

 engineers. But as engineers in the broader sense, we are as 

 much interested in questions of recondite physics as of costs of 

 generation. 



To sum up as to the arc light, we do not seem to have 

 reached our limit as to light from pure heating, because we lose 

 a lot of light into the opposite carbon. Many attempts have 

 been made to expose the crater freely. But, far more important 

 than this, I would urge that the arc is not necessarily a hot body 

 radiator only, but. that it may also convert electrical power 

 directly into light in the space between the electrodes, and this 

 gives a chance of rising more nearly to our theoretical limit of 

 about five candles per watt. 



The Incandescent Lamp. 



This simple hot carbon wire in a bulb involves the most 

 extraordinary physical complexities. A great many curious 

 things go on inside the simple-looking globe. A good account 

 of what is known— especially since he took the subject in hand 

 — has been written by Dr. Fleming, and the scientific manufac- 

 ture of this interesting article has been fully described by Mr. 

 Ram. The incandescent lamp is a simple hot body radiator, and 

 the limit of efficiency depends chiefly on the temperature of the 

 carbon. As we are limited by the size of mains, we can only 

 use pressures of ioo volts or 200 volts, and this limits us to 

 carbon or something of still higher specific resistance. The 

 sensitiveness of the carbon lamp to pressure in its turn limits the 

 practical variation of pressure of supply, and thus costs us very 

 heavily in mains. If we had incandescent lamps which did not 

 mind 20 per cent, pressure variation, we would have saved 

 millions in mains in this country alone. 



The idea of making lamps of carbides has become very 

 fashionable lately. People have put oxides into carbon for the 

 last twenty years. The old idea is to get hold of an oxide that 

 radiates more light at a given temperature than it ought to, 

 which is itself a fallacy, while the idea of oxide in contact with 

 carbon is chemically absurd. There is no oxide irreducible by 

 hot carbon. The carbides are not by any means all refractory. 

 Some are, though, but there are immense difficulties in making 

 carbide lamps. To make a fine filament material of an infusible 

 material, which can be made only at electric furnace tempera- 

 tures and is generally decomposed by moist air, is not an easy 

 task. It is easy to think you have made a carbide lamp by 

 incorporating an oxide in the filament material, but the 

 resulting filament is generally mostly, if not wholly, carbon. 

 What happens to the metal in the circumstances is rather a 

 mystery. There is, however, a chance of enlarging our limits 

 in incandescent lamps of the ordinary kind, but it seems strange 

 that the melting points of all known materials should suddenly 

 reach a higher limit. Assuming the Stefan-Boltzmann law for 

 ordinary light radiations, the fact that the efficiencies of 

 refractory bodies all reach limits of the same order shows that 

 the most refractory bodies melt at about the same temperature, 

 somewhere in the neighbourhood of 3000° A. Whatever the 

 inter-molecular forces may be that bind the particles to make 

 solids, the vibration forces due to temperature seem to overcome 

 the greatest at about 3000°. 



Instead of an ordinary conductor, Nernst uses an electrolyte 

 which stands a higher temperature. The conduction is electro- 

 lytic, as can easily be shown, but there are many curious pheno- 

 mena, many of them so far unexplained, in the Nernst lamp. 

 The efficiency of the Nernst lamp is about 0'6 candle per watt. 

 It was at one time supposed to owe its efficiency to selective 

 emission, but there is no reason to doubt that it is a pure 

 temperature radiation. 



NO. 1729, VOL. 67] 



Electric Heating. 

 The limit of electric heating is clearly purely financial. To 

 convert heat into other energy with a very small efficiency anc 1 

 to send it out by expensive cables and then to degrade the 

 energy down to heat again is obviously much dearer than 

 burning coal or gas direct. But in many domestic cases, the 

 convenience is so great that the limit is not so low as might be 

 thought, and electric heating for cooking and other domestic 

 uses may develop considerably. The electric arc and incan- 

 descent lamps are essentially cases of electric heating. By far 

 the most important use of electric heating is the furnace. Here 

 the temperature available is only limited by the volatilisation of 

 the electrodes, and this enables us to get temperatures other- 

 wise unavailable, so that we can get chemical actions which are 

 impossible at lower temperatures, either because they are endo- 

 thermic or because the materials do not come into chemical 

 contact at ordinary temperatures. It is impossible to say what 

 our limits are in the electrical furnace. Probably the tempera- 

 ture is limited by the volatilising of carbon. The products are 

 not limited to endothermic compounds ; the furnace is useful for 

 the reduction of metals and phosphorus, and for melting glass 

 and, it is hoped, silica for optical and laboratory purposes, and 

 perhaps for cooking utensils and evaporating pans and crucibles 

 in chemical engineering and metallurgy. 



Railways. 



It is almost absurd to begin to consider the limits of the use 

 of electrical transmission on railways at this date. The future of 

 electric railways, electric tramways and automobiles is rather a 

 matter of vague conjecture and picturesque prophecy. Tubes 

 are multiplying rapidly, and railways are putting down electric 

 transmission on suburban lines in Europe and the States. On 

 short lines with many stops, we have to contend with inefficiency 

 at starting. On long lines, there is difficulty of transmission or 

 cost of transformation and difficulties of collection. We are 

 limited by the want of either a variable speed simple alternate- 

 current motor or a simple variable speed-gear capable of trans- 

 mitting a very large torque and packing into an engine. A 

 recently developed scheme is the use of low-frequency alter- 

 nating currents with laminated series-wound motors. This 

 solves the difficulty, but at the expense of large idle current, 

 induced pressure in short-circuited armature coils, large expen- 

 sive and inefficient transformers, and the ordinary disadvantages 

 of the series-motor on constant pressure. This plan is well 

 worth serious study. 



The collection of large currents at great speeds has long 

 loomed as a limit. The published accounts of experiments at 

 Zossen would lead us to suppose there is no trouble on this score. 

 Still, it is a difficulty many engineers fear. 



In electric tramways, there is no limit in sight. The power 

 can be sent over any distance desired, and there seems to be 

 no limit to the people who want to travel on electrical trams. 

 The question of electrolysis is rather that of a limit to the dura- 

 tion of pipe companies' property. It is a very difficult question. 

 Though the threatened effects of electrolysis have no doubt been 

 exaggerated, it is at best a question of degree, and the ingenuity 

 of engineers is continually reducing the chance of damage. It 

 has recently been urged that frequent reversals of polarity of 

 the system reduce the electrolysis very considerably. 



Electrolysis, 



This is a branch of industry in which it is very difficult to tell 

 our limits. In electrolytic copper-refining, our limit is that of 

 the copper wanted. Our electrolytic industries suffer mostly 

 from the limits of intelligence of the investing public. It is 

 assumed that we cannot do electrolysis in England because we 

 have no water power. This is only an excuse for inactivity. 

 As already explained, we can do just as well without water 

 power. A blast furnace is much more valuable than a waterfall 

 of similar power, because it is near coal and in an industrial 

 district. Moreover, as already explained, the cost of electrical 

 energy is a small portion of that of most electrolytic products. 

 At first, electrolysis was to be applied to copper-refining. Then 

 to caustic soda. The output of electrolytic caustic is really 

 rather limited by the demand for bleach. What is urgently 

 wanted is some other way of storing and carrying chlorine. 

 Steel bottles and compression plant are an unsatisfactory solu- 

 tion. What are the limits in the way of electrolysing fused salt 

 They are all incidental limits. The containing vessel is 



