.S7A> WILLIAM SIEMENS F.R.S. 393 



developing electric energy out of heut. Sir William Armstrong, 

 in his inaugural address at York in 1881, as President of the 

 Mechanical Section of the British Association, drew particular 

 attention to the thermo-battery, as one of the most hopeful sources 

 of ultimate electrical effect, and physical experimentalists should 

 lose sight of this interesting problem. Yet the thermo- 

 liauu-y has one drawback, in common with the steam-engine or 

 any thermo-motor that is, it is dependent, not only on the first 

 law of thermo-dynamics, according to which heat is changed 

 entirely into its equivalent of electricity, but also on the second 

 law, which says that whenever such conversion of heat takes place, 

 a certain amount of heat must descend from a point of high- 

 potential to a point of low-potential. It is thus that our best 

 steani-engines give in mechanical force only about one-seventh of 

 the theoretical equivalent of the heat-energy ; and it is owing to 

 this second law of thermo-dynarnics that there must be necessarily 

 a loss of heat, by conduction in the metal strips themselves, which 

 conducted heat must be abstracted at the cooled extremities all 

 round, in order to keep up the extremes of temperature upon 

 which the action of the thermo-battery depends. We will now 

 see whether we can produce a visible effect by the current on the 

 electro-dynamometer, an instrument the nature of which I shall 

 have occasion to describe hereafter. The action is not great, but 

 you see that there is a very decided deflection to this side of about 

 5 degrees. The battery has not been on long, or it would probably 

 amount to 10 degrees. From measurements which I have only 

 lately made at leisure, I find that it would require one thousand 

 eight hundred single pairs of these strips to produce a potential 

 sufficient to work an incandescent electric light, showing how very 

 slight the current really is. 



I now approach a subject in our lecture which is of the greatest 

 importance. I have here the original magnet, and the original 

 coil, which Faraday used in the year 1831 fifty-two years ago 

 to develop the first induction spark. In 1826, or 1827, he had 

 already conceived the idea that when an armature was removed 

 forcibly from a permanent magnet, the expenditure of force should 

 give rise to a current in the wire surrounding the armature ; but 

 it took him three or four years to develop the idea. When Fara- 

 day saw the spark, and was able to show it to the members of the 



