7 
Professor Leslie on Electrical Theories. 
a discharging rod, and hold it, by means of the insulated handle, 
within half a foot or so of the prime conductor, and while the 
machine is worked, small sparks may be perceived on touching 
the extremity of the other branch, which is stretched to its 
greatest distance ; or resting it on the knob of a small phial, a 
charge may be given. If a smaller ball be substituted, the 
sparks will become more vivid, and succeed each other faster. 
But when it is at last contracted to a point, the effects will be 
vastly increased. 
Since the distinction, then, between the point and the knob 
is not absolute, may we not presume, from the former investi- 
gations, that it arises entirely from a difference in the force or 
quantity of the aerial current ? It would thence follow, that if, 
by any contrivance, we can produce an equal stream at a ball, 
we shall give it the property for which an acute body is distin- 
guished, Let us employ the simplest and most effectual me- 
thod ; that of rarifying the air by means of heat. Screw a red- 
hot metal ball, of about an inch diameter, to the end of one of 
the branches of the discharging rod, and hold it at the distance 
of a foot from the prime conductor ; on turning the cylinder, 
very intense sparks may be obtained at the other branch, and 
the effect will, according to circumstances, be equal, or even su- 
perior, to that of a point. But, as the ball cools, the sparks 
will grow weaker and weaker, and at last will become hardly 
perceptible. If we now stick a bit of lighted taper on the end 
of a glass tube, and bring it close to the ball, the effect will in- 
stantly be renewed ; and, by thus removing or approaching the 
is the greatest , and it diminishes as they approach to parallelism , The compound 
point thus differs not essentially in its action from a knob. 
The effect of these various modifications is merely to retard or check the flow of 
electrified air about a point ; but., if such an afflux by any means be considerably aug~ 
mented round a knob, this will approximate to the property of a point . Thus, tie a 
bit of lighted taper close to the knob , and it will be found to arrest the sparking near - 
ly at the same distance as the simple point. The flame likewise will appear violently 
blown towards the knob. To vary this experiment , take a thick wire, 12 or 15 
inches long, and terminated by a solid ball of iron, an inch in diameter , and 
having heated the ball to redness, advance it toivards the conductor, and the sparking 
will cease ; but , as the ball cools, it must be drawn nearer to produce the same ef° 
feet. 
