704 
Designate the right-hand paper sector by a, 
the brush connected with it by b, the comb 
connected with the discharging-rod by c, the 
comb in metallic connection with a similar 
comb diagonally opposite by d, and the corre- 
sponding parts on the left-hand side by a’, 
One, and d’. 
If the left-hand sector a’ be charged with 
negative electricity, it draws positive electricity 
from the comb d’ upon the revolving-plate, 
which is supposed to move in the direction of 
the hands of a watch. When the positive 
electricity on the plate reaches the brush 8, it 
draws negative electricity from the brush, and 
leaves the sector a charged with positive. 
This positive electricity on the sector, and 
the positive that is left on the plate, both draw 
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negative from the comb c, and leave the dis- 
charging-rod connected with it charged with 
positive electricity. 
The plate, now nearly neutral, passes under 
the diagonal comb d, and from it receives a 
charge of negative electricity, drawn out by 
the positive on the sector a, which at the same 
time repels positive electricity along the diago- 
nal, and out of d’ upon the revolving-plate. 
The plate, now charged with negative elec- 
tricity, passes under the brush 0’, draws posi- 
tive electricity from it, and thus increases the 
negative charge on the sector a’. The residue 
of negative electricity on the plate, and the 
negative on the sector, both draw positive from 
the comb c’, and leave the discharging-rod 
connected with it charged with negative elec- 
tricity. The plate, now nearly neutral, passes 
SCIENCE. 
[Vou. III., No. 72. 
to our starting-point d’, and the process is 
repeated. 
If the machine is operated in the dark, the 
brush b’, and the points on c’ and d’, will be 
tipped with the well-known positive brushes, 
while 6, c, and d will show only the negative 
slowing points. The relative lengths of the ~ 
brushes on c’ and d’ will depend on the position 
of the discharging-rods. Ifa is negative in 
the place of a’, the position of the brushes 
will, of course, be reversed. The nature of 
the electricity on any part of the machine may 
be tested by bringing the point of a lead- 
pencil near it, and noticing the form of the » 
discharge from the point. 
The great use of the knobs on the revolving- 
plate is to keep the paper sectors charged. 
Two knobs give better results than the 
usual six or eight; the reason appar- 
ently being, that the larger number of 
knobs keeps the sectors overcharged, 
and there is a continual loss (by brushes 
from the sectors) of electricity that 
would otherwise remain on the revolv- 
ing-plate, and help to increase the 
charge on the discharging-rods. 
Somewhat longer sparks are also ob- 
tained by connecting the sectors and 
discharging-rods on the same side by 
conductors ; but the machine, thus ar- 
ranged, reverses more readily, does not 
give sparks so rapidly when the dis- 
charging-knobs are near together, and 
is started by separating the knobs. 
In some forms of the machine used 
in Germany the fixed plate is in two 
parts, separated by a vertical air-space. 
This is an improvement, because it 
prevents the electricities of the two 
sectors from uniting across the surface 
of the plate. Machines of this sort sometimes 
have as many as sixty revolving-plates, all on 
one axis. Such machines give large quantities 
of electricity, but not very long sparks. 
The following experiments, not generally 
known, illustrate the power of the single plate 
machine. If a strip of vulcanite about two 
inches wide is moved to and fro in front of 
the positive pole, the length of the spark will 
be greatly increased, sometimes reaching five 
inches and a half on a machine whose revolv- 
ing-plate is only ten inches and a half in 
diameter. If a drop of stearine is placed on 
a thin sheet of glass, which is too large for the 
spark to pass around the edges, and the glass 
is held between the discharging-knobs of a 
good ten-and-a-half-inch machine, with the 
drop toward the positive knob, the spark will 
