6 
Professor Leslie on Electrical Theories . 
three inches diameter, and made to project on the other side a 
quarter or half an inch. Let the point, carrying its card, be 
advanced towards the prime conductor till it become lucid ; take 
off the card, and the point will shew its bright spangle, though 
now withdrawn to double or triple its former distance. By this 
little contrivance, the air remains heaped about the card, and 
its renewal at the point is obstructed 
Nor are the properties of the point and knot radically dis- 
tinct ; they differ only in degree. For, fix a smooth metal ball 
of above an inch diameter to the end of one of the branches of 
* The mode which the author common ly employs in repeating this set of experi- 
ments ~is simpler , and better fitted for exhibition to a large auditory. A brass ball of 
two inches in diameter , being fixed to the top of a glass pillar , is connected , as occa- 
sion requires , by a chain or wire , with the table and the ground. Another similar 
ball , of the diameter of an inch , is screwed to the end of a thick wire , of near six 
inches in length, mid tapering to a point. Note, place the ball , with its stand , at the 
sparking distance , or about two or three inches before the prime conductor , and op- 
posite to the farther end. Turn the machine regularly , and the sparks will succeed 
each other at equal intervals. While this action is continued , grasp the tapered por- 
tion of thick wire in the hand , and bring the knob gradually nearer the conductor ; 
no alteration of effect will be perceived till the knob has approached perhaps within 
two inches , when the sparks will strike it , instead of the ball. But , reverse the po- 
sition of the thick wire , and , holding by the knob , present the tapering and pointed 
end to the conductor , at the distance of above afoot , the sparks to the ball will be- 
come less frequent , but , advancing within half a foot , they will cease altogether. In 
this situation , apply a finger to the point of the wire , and the sparks will strike as 
before ; and , as often as the finger is removed or replaced , they ivill stop or revive. 
Plant a square screen of tinfoil , nearly of the same length as the prime conduc- 
tor , about half afoot in front of it. While the machine is kept working , approach the 
tapered point towards the screen , and the electrical sparks will now continue without 
alteration ; but , advance the point between the nearer edge of the screen and the 
conductor , and the sparks will cease. Cut a hole near the middle of the tinfoil , of 
about half an inch in diameter, and bring the point an inch behind it ; the sparks 
will return with their former intensity , nor will they stop till the pointed wire has 
passed through the hole , and advanced more than half an inch beyond it. Remove 
this screen, and thrust the wire a quarter or half an inch through the centre of a 
card, formed into a circle of three inches in diameter . The property of the point to 
draw off the electrical influence will, by this modification , be completely destroyed. 
The point must be pushed near an inch beyond the surface of the card, to enable it 
to recover its influence . While the tapering point is held at the distance where it 
arrests the sparks darted from the prime conductor, advance gradually the other hand 
with a similar point, and, when the interval is reduced to about half an inch, the 
sparking will again recommence. When the pointed ends are opposite, the interval 
