462 FRAGMENTS OF SCIENCE 



The Farmer- Wallace machine is also an apparatus of 

 great power. It consists of a combination of bobbins for 

 induced currents, and of inducing electro-magnets, the 

 latter being excited by the method discovered by Siemens 

 and Wheatstone. In the machines intended for the pro- 

 duction of the electric light, the electro -motive force is so 

 great as to permit of the introduction of several lights in 

 the same circuit. A peculiarly novel feature of the Farmer- 

 Wallace system is the shape of the carbons. Instead of 

 rods, two large plates of carbons with bevelled edges are 

 employed, one above the other. The electric discharge 

 passes from edge to edge, and shifts its position according 

 as the carbon is dissipated. The duration of the light in 

 this case far exceeds that obtainable with rods. I have 

 myself seen four of these lights in the same circuit in 

 Mr. Ladd's workshop in the City, and they are now, I 

 believe, employed at the Liverpool Street Station of 

 the Metropolitan Eailway. The Farmer- Wallace '* quan- 

 tity machine" pours forth a flood of electricity of low 

 tension. It is unable to cross the interval necessary 

 for the production of the electric light, but it can fuse 

 thick copper wires. When sent through a short bar 

 of iridium, this refractory metal emits a light of extraor- 

 dinary splendor. 1 



The machine of M. de M&itens, which he has gener- 

 ously brought over from Paris for our instruction, is the 

 newest of all. In its construction he falls back upon the 

 principle of the magneto -electric machine, employing per- 

 manent magnets as the exciters of the induced currents. 



1 The iridium light was shown by Mr. Ladd. It brilliantly illuminated the 

 theatre of the Royal Institution. 



