8 Professor Tyndall [Jan. 17, 



Brethren to introduce this powerful source of illumination with all 

 its recent improvements at certain prominent points on the coast of 

 England." With regard to the application of electricity to light- 

 house purposes, the course of events was this : The Dungeness light 

 was introduced on January 31, 1862; the light at La Heve on 

 December 26, 1863, or nearly two years later. But Faraday's ex- 

 perimental trial at the South Foreland preceded the lighting of 

 Dungeness by more than two years. The electric light was after- 

 wards established at Cape Grisnez. It was started at Souter Point on 

 January 11, 1871 ; and at the South Foreland on January 1, 1872. 

 At the Lizard, which probably enjoys the newest and most powerful 

 development of the electric light, it began to shine on January 1, 1878. 



I have now to revert to a point of apparently small moment, but 

 which really constitutes an important step in the development of this 

 subject. I refer to the form given to the rotating armature in 1857 by 

 Dr. Werner Siemens, of Berlin. Instead of employing coils wound 

 transversely round cores of iron, as in the machine of Saxton, 

 Siemens, after giving a bar of iron the proper shape, wound his wire 

 longitudinally round it, and obtained thereby greatly augmented 

 effects between suitably placed magnetic poles. Such an armature is 

 employed in the small magneto-electric machine which I now intro- 

 duce to your notice, and for which the Institution is indebted to 

 Mr. Henry Wilde, of Manchester. There are here sixteen permanent 

 horse-shoe magnets placed parallel to each other, and between their 

 poles a Siemens' armature. The two ends of the wire which sur- 

 rounds the armature are now disconnected. In turning the handle 

 and causing the armature to rotate, I simply overcome ordinary 

 mechanical friction. But the two ends of the armature coil can be 

 united in a moment, and when this is done, I immediately expe- 

 rience a greatly increased resistance to rotation. Something over 

 and above the ordinary friction of the machine is now to be over- 

 come, and by the expenditure of an additional amount of muscular 

 force I am able to overcome it. The excess of labour thus thrown 

 upon my arm has its exact equivalent in the electric currents gener- 

 ated, and the heat produced by their subsidence in the coil of the 

 armature. A portion of this heat may be rendered visible by con- 

 necting the two ends of the coil witli a thin platinum wire. When 

 the handle of the machine is rapidly turned the wire glows, first 

 with a red heat, then with a white heat, and finally with the heat of 

 fusion. The moment the wire melts, the circuit round the armature 

 is broken, an instant relief from the labour thrown uj^on the arm being 

 the consequence. Clearly realize, I beg of you, the equivalent of the 

 light here developed. During the j^eriod of turning the machine a 

 certain amount of combustible substance was oxidized or burnt in the 

 muscles of my arm. Had it done no external work, the matter 

 cousutnod would have produced a definite amount of lieat. Now, the 

 muscular heat actually developed during the rotation of the machine 



