16 
substituting a slip of plaster of Paris for the cartridge and 
partition of kaolin formerly employed. 
To produce the alternating currents, however, to supply a 
number of lights, it was found necessary to employ power- 
ful electro magnetic induction machines, excited by the 
currents from other smaller machines, according to the 
principles laid down in my paper read before the Royal 
Society, and published in the Philosophical Transactions of 
1867. From 16 to 20 lights are produced from one of these 
electro magnetic machines, each light absorbing about one 
horse power. 
The system of electric lighting above described would 
now seem to be definitely established in some places as a 
substitute for gas, and visitors to the French capital during 
the present summer, will have been struck with the fine 
effects produced in the avenues and squares where the light 
is displayed. 
My connection with the history of this system of lighting, 
placed me in a position to make some experiments with the 
Jablochkoff candle, and led to the discovery of the following 
facts. One of the conditions necessary for producing a con- 
stant light from the candle, in its most recent form, was 
that the quantity and intensity of the alternating current 
should be such, that the carbons consume at a rate of from 
four to five inches per hour. If the electric current were 
too powerful, the carbons became unduly heated, and pre- 
sented additional resistance to the passage of the current; the 
points at the same time lost their regular conical form. If, 
on the other hand, the current were too weak, the electric 
arc played about the points of the carbons in an irregular 
manner, and the light was easily extinguished by currents 
of air. 
In the course of these experiments I was struck with the 
apparently insignificant part which the insulating material 
played in the maintenance of the light between the carbon 
