THE ELECTRIC LIGHT. 677 
furnishes a direct current, which is carried round the 
electro-magnets of a second and larger machine. Wilde's 
principle, it may be added, is also applied on the Thames 
Embankment and the Holborn Viaduct; a small Gramme 
machine being used in each case to excite the electro- 
magnets of the large one. 
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 
production 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. In- 
stead of rods, two large plates of carbons with beveled 
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 Railway. The Farmer-Wallace 
" quantity 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 extraordinary 
splendor.* 
The machine of M. de Meritens, 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 
permanent magnets as the exciters of the induced currents. 
Using the magnets of the Alliance Company, by a skillful 
disposition of his bobbins, M. de Meritens produces with 
eight magnets a light equal to that produced by forty mag- 
nets in the Alliance machines. While the space occupied 
is only one-fifth, the cost is little more than one-fourth 
*The iridium light was shown by Mr. Ladd. It brilliantly illu- 
minated the theater of the Royal Institution. 
