On the Generation of Statical Electricity. 111 
tension as that of the glass, without throwing any of it off. 
“This tension cannot be exceeded.’* If of greater size, they 
are injurious by reason of their increased surface, presenting an 
increased extent to the conducting action of the air. If of small 
size, unless spherical, the excess of electricity is rapidly thrown 
off; in charging an electrical battery, however, they are prefera- 
ble to those of larger size. 
Prime conductors probably have the best form for common 
purposes when they are composed of two branches, each having 
a length double that of the cylinder, (or equal to the diameter of 
the plate, ) and a diameter one fourth that of the cylinder, or one 
tenth that of the plate: connected by a cross piece one half the 
diameter of the conductors. Brass smoothly gilt is the best 
ordinary material; it should be neither varnished nor painted, 
for the sparks break through such coatings, leaving rough points, 
which are sometimes highly detrimental.+ 
The only part requiring particular attention on the subject of 
the prime conductors, is their points to receive the electricity of 
the glass. A common pin is about one inch long and one twen- 
tieth of an inch in diameter; let it be supposed that both ends 
are pointed and covered with little balls of wax; apply it to an 
excited body; it will receive a certain charge which has a ten- 
dency to escape, which tendency, for a unit of surface equal to 
that of the point of the pin, at the central portions, will be repre- 
sented by unity. Remove the wax balls; the pin may now be 
considered a prolate spheroid, whose transverse axis is one inch 
in length, and its conjugate, one twentieth of an inch. But in 
such case the tendency to escape at the central portions of the 
spheroid, is to the same effort at the extremities, ‘as the square 
of the conjugate axis (1*) is to the square of the transverse 
(202) :’{ hence the effort to escape at the point of the pin is 
represented by 400. Let it be now supposed that the pin retains 
its point but doubles its own diameter ; the proportion will then 
be as 2? to 202, or as 1 to 100. Hence by doubling the diame- 
ter of the pin, it has diminished the power of its point to one 
fourth ; this shows the importance of having slender points. 
The influence of a point depends moreover on the tension of 
the electricity, and appears to act as follows. From the position 
*M. pee Vol. II, p. 205. 
+ Faraday’s Chem. Manip., note by Prof. Mitchell, p. 452. 
} Murphy’s Marhesiiaiell Discussions on Eleciricity, p- 69. 
