Sept. 8, 1881] 



NATURE 



451 



the cell of a voltaic battery, and we may multiply the.n to any 

 extent, and group them in series or in parallels, with the same 

 results as are obtained by similar combinations of voltaic cells. 

 The electricity so produced we term Thermo-electricity, and the 

 apparatus by which the current is evolved is the thermo-electric 

 battery. At present this apparatus is even more wasteful of heat 

 than the steam-engine, but considering the very recent origin of 

 this branch of electrical science, and our extremely imperfect 

 knowledge of the actions involved, we may reasonably regard 

 the present thermo-electric battery as the infant condition of a 

 discovery, which, if it follow the rule of all previous discoveries 

 in electricity, only requires time to develop into great practical 

 importance. Now if we possessed an efficient apparatus of this 

 description we could at once apply it to the steam-engine for the 

 purpose of converting into electric energy the heat which now 

 escapes with the rejected steam, ani the gases from the fire. 

 The vice of the steam-engine lies in its inability to utilise heat of 

 comparatively low grade, but if we could use up the leavings of 

 the steam-engine by a supplemental machine acting on thermo- 

 electric princijiles, the present excessive waste would be avoided. 

 We may even anticipate tint in the distant future a thermo- 

 electric engine may not only be used as an auxiliary, but in com- 

 plete substitution of the steam-engine. .Such an expectation 

 certainlj' seems to be countenanced i)y what we may observe in 

 animated nature. An animal is a Jiving m.achine dependent 

 upon food both for its formation and its action. That portion 

 of the food which is not used for growth or structural repair, acts 

 strictly as fuel in the production of heat. Part of that heat goes 

 to the maintenance of the animal temperature, and the remainder 

 gives rise to mechanical action. The only analogy between the 

 steam-engine and this living engine is that both are dependent 

 upon the combustion of fuel, the combustion in the one case 

 being extremely slow, and in the other very rapid. In the 

 steam-engine the motiin is produced by pressure, but in the 

 animal machine it is effected by mu cular contraction. The 

 energy which causes that contraction, if not purely electrical, is 

 so much of that nature that we can produce the same effect by 

 electricity. The conductive system of the nerves is also in 

 harmony with our conception of an electrical arrangement. In 

 fact a description of the animal machine so closely coincides 

 with that of an electrodynamic machine actuated by thermo- 

 electricity, that we may conceive them to be substantially the 

 same thing. At all events, the animal process begins with com- 

 bustion and ends with electrical action, or something so nearly 

 allied to it as to differ only in kind. And now observe how 

 superior the result is in nature's engine to what it is in ours. 

 Nature only uses heat of low grade, such as we find wholly un- 

 available. We reject our steam, as useless, at a temperature 

 that would cook the animal substance, while nature works with 

 a heat so mild as not to hurt the most delicate tissue. And yet, 

 notwithstanding the greater availability of high-grade tempera- 

 ture, the quantity of work performed by the living engine rela- 

 tively to the fuel consximed, puts the steam-engine to shame. How 

 all this is done in the animal organisation we do not yet under- 

 stand, but the result points to the attainability of an efficient 

 means of converting low-grade heat into electricity, and in 

 striving after a method of accomplishing that object we shall 

 do well to study nature, and profit by the excellence which is 

 there displayed. 



But it is not alone in connection with a better utilisation of 

 the heat of combustion that thermo-electricity bears so important 

 an aspect, for it is only the want of an efficient apparatus for 

 converting heat into electricity, that prevents our using the direct 

 heating action of the sun's rays for motive power. In our 

 climate, it is true, we shall never be able to depend upon sun- 

 shine for power, nor need we repine on that account so long as 

 we have the preser\'ed sunbeams which we possess in the con- 

 densed and portable form of coal, but in regions more favoured 

 with sun and less provided with coal the case would be different. 

 The actual power of the sun's rays is enormous, being computed 

 to be equal to melting a crust of ice 103 feet thick over the whole 

 earth in a year. Within the tropics it would be a great deal 

 more, but a large deduction would everywhere have 1 1 be made 

 for absorption of heat by the atmosphere. Taking all things 

 into account, however, we shall not be far from the truth in 

 assuming the solar heat, in that part of the world, to be capable 

 of melting annually, at the surface of the ground, a layer of ice 

 85 feet thick. Now let us see what this means in mechanical 

 effect. To melt l lb. of ice requires I42'4 English units of heat, 

 which, multiplied by 772, gives _us 109,932 foot pounds as the 



mechanical equivalent of the heat consumed in melting a pound 

 of ice. Hence we find that the solar heat, operating upon an 

 area of one acre, in the tropics, and competent to melt a layer of 

 ice 85 feet thick in a year, would, if fully utilised, exert the 

 amazing power of 4000 horses acting for nearly nine hours every 

 day. In dealing with the sun's energj- we could afford to be 

 wasteful. Waste of coal means waste of money and premature 

 exhaustion of coal-beds. But the sun'-, heat is'poured upon the 

 earth in endless profusion — endless at all events in a practical 

 sense, for whatever anxiety we may feel as to the duration of 

 coal, we need have none as to the duration of the sun. We 

 have therefore only to consider whether we can divert to our use 

 so much of the sun's motive energv- as will repay the cost of the 

 necessary apparatus, and whenever such an apparatus is forth- 

 coming we may expect to bring into subjection a ver)' consider- 

 able proportion of the 4000 invisible horses which science tells us 

 are to lie found within every acre of tropical ground. 



But u hatever may be the future of electricity as a prime mover, 

 either in a dominant or subordinate relation to heat, it is certain 

 to be largely used for mechanical purposes in a secondary capa- 

 city, that is to say, as the offspring instead of the parent of 

 motive power. The most distinctive characteristic of electricity 

 is that which we expre-s by the word "current," and this gives 

 it great \alue in cases where power is required in a transmissible 

 form. The term may be objected to as implying a motion of 

 translation analogous to the flow of a liquid through a pipe, 

 whereas the passage of electricity through a conductor must be 

 regarded as a wave-like action communicated from particle to 

 particle. In the case of a fluid current through a pipe, the 

 resistance to the flow increases as the square of the velocity, 

 while in the case of an electric current the resistance through a 

 given conductor is a constant proportion of the energy trans- 

 mitted. .So far therefore as resistance is concerned electricity 

 has a great advantage over water for the transmission of power. 

 The cost of the conductor will however be a grave consideration 

 where the length is great, because its section must be increased 

 in proportion to the length to keep the resistance the s.ame. It 

 must also be large enough in section to prevent heating, which 

 not only represents loss but impairs conductivity. To work 

 advantageously on this system, a high electromotive force must 

 be used, and this will involve loss by imperfect insulation, in- 

 creasing in amount with the length of the line. For these 

 reasons there will be a limit to the distance to which electricity 

 may be prifitabiy conveyed, but within that limit there will be 

 wide scope for its employment transmissively. Whenever the 

 time arrives for utilising the power of great waterfalls the trans- 

 mission of power by electricity will become a system of vast 

 importance. Even now small streams of water inconveniently 

 situated for direct application may, by the adoption of this 

 principle, be brought into useful operation. 



For locomotive purposes also we find the dynamo-electric 

 principle to be available, as instaiiced in the very interesting 

 example presented in Siemens' electric railway, which has 

 already attained that degree of success which generally fore- 

 shadows an important future. It forms a combined fixed engine 

 and locomotive system of traction, the fixed engine being the 

 generator of the power and the electric engine representing the 

 locomotive. 



Steam power may both be transmitted and distrilrated, by the 

 intervention of electricity, but it will labour under great disad- 

 vantage w hen thus applied, until a thoroughly effective electric 

 accumulator be provided, capable of giving out [electric energ}' 

 with almost unlimited rapidity. How far the secondary batter\- 

 of M. Faure will fulfil the necessary conditions remains to be 

 seen, and it is to be hoped that the discussions which may be 

 expected to take place at this meeting of the British Association 

 will enable a just estimate of its capabilities to be formed. The 

 introduction of the F.aure battery is at any r.ite a very important 

 step in electrical progress. It will enable mo'ors of small fiower, 

 whatever their nature may be, to accomplish, by uninterrupted 

 action, the effect of much larger machines acting for short periods, 

 and by this means the value of very small streams of water will 

 be greatly erdianced. This will be especially the case where the 

 power of the stream is required for electric lighting, which, in 

 summer, when the springs are low, will only be requn'ed during 

 the brief hours of darkness, while in winter the longer nights will 

 be met by a more abundant supply of water. Even the fitful 

 power of wind, now so little used, will probably acquire new 

 life when aided by a system which will not only collect, but 

 equalise, the variable and uncertain power exerted by the air. 



