THE ELECTRIC LIGHT. 
29 
pated by Mr. Chapman’s patent, dated more than twenty-three 
years ago. 
Among the more modern systems worthy of notice are the 
Rapieff, the Werdermann, and the Wallace-Farmer. 
The last of these has been brought into prominent notice 
by its connection with Mr. Edison’s still undisclosed discovery.* 
The Wallace-Farmer dynamo-electric machine divides American 
patronage with the Brush machine, just as the Siemens machine 
divides the English market with the Gramme. It comprises a 
dynamic machine as well as a special form of lamp. The lamp 
consists of a metal frame of brass, carrying two carbons in the 
form of slabs about nine inches long and three inches broad ; 
the upper, or positive, being about half an inch thick ; the lower, 
or negative, only about a quarter of an inch. The lower carbon 
is fixed to the bottom of the frame ; the upper is carried by a 
cross-piece sliding up and down in side grooves. It can, there- 
fore, be drawn apart from the lower to any adjusted distance, 
so as to determine the luminous arc. When the lamp is not in 
use, the upper carbon is let down into contact with the lower ; 
but the act of putting on the current raises it one-eighth of 
an inch, and establishes the light by means of an ingenious 
mechanism. The arc either starts at the point of least resist- 
ance, or may be started by inserting a metal conductor for a 
moment between the t wo carbon edges. Once started, it subsists 
at that point until the consumption of carbon widens it to such 
a degree that a shorter and less resisting path for the current is 
to be found at a neighbouring point. In this way the arc 
travels slowly along the whole edge of the carbon, and when it 
reaches the other end, it turns and comes back again. The 
light can thus be maintained for one hundred hours without 
change of carbon, at a cost of about a penny per hour. 
There can be no doubt that where exact stability in position 
of the luminous arc is not essential, this is a very cheap and 
simple method of producing the light. Messrs. Ladd & Co., 
who are the sole British agents, have exhibited it at their 
Shoreditch manufactory, terming it the 44 Workman’s Lamp.” 
The Wallace-Farmer duplex dynamo-electric machine con- 
tains twenty-five coils set round the armature, each coil wound 
with four separate wires, the ends of which are brought up to 
an axial commutator, so that there are one hundred 44 makes 
and breaks ” in one revolution. It can be worked up to 800 
revolutions per minute, the power required to drive it being 
about one hundred revolutions per minute for each horse-power ; 
each hundred, or each horse-power, being sufficient for one 
lamp. The armature, electro-magnets, and the work to be done 
are all in one circuit. 
* “ Telegraphic Journal,” Nov. 7, 1878. 
