58 Observations on some Experiments in Electricity. 
consists of a dark spot occurring in the luminous track of the spark, 
when it passes between two balls or obtusely pointed rods. The longer 
the spark can be drawn, provided it pass in a single track, the more 
readily will the point of darkness (which, for distinction, we may term 
the neutral point) be detected by the eye. It will be found in dif- 
ferent parts of the course, according to the relative magnitudes and 
other circumstances of the “ positive” and “negative” balls, between 
which the spark passes. A commodious arrangement for exhibiting 
the phenomenon of the neutral point, is to insulate, on glass stands, 
two rods terminated with balls, and then making a connection of one 
with the prime conductor, and of the other with the rubber of the 
machine, to set the latter in motion, adjusting the distance of the two 
balls apart, to the power of the machine, state of soa &c. so as 
to yield the longest possible sparks. 
A rapid succession of sparks may thus be aa to pass, and the 
neutral point will be the more readily perceived, the more nearly 
continuous is their repetition. 
When one ball is much larger than the other, the dark Spot is 
generally nearer to the larger ball. 
e spark, if not more than three or four inches in length, will 
in general be perceived to traverse a path nearly direct, between the 
nearest points of the two balls; but not unfrequently the two Jumin- 
ous sections appear to meet after traversing the air in directions 
somewhat divergent from the straight line joining the centres of the 
balls. Here the neutral spot will be found. At other times, the 
two sections appear to follow lines, parallel to each other but not co- 
incident, as if two streams were rushing in opposite directions and 
about to pass each other, when both are arrested and extinguished 
at the neutral point, without ‘actually overlapping each other to any 
perceptible extent. It has probably been observed by others that 
the spark frequently assumes the aspect of a double cone of light, 
the base of each resting on one of the balls or other conducting 
bodies, and the two apexes united forming a variety of colors, blue, 
purple, &c, This appearance may occur when the balls are mach 
nearer than the maximum striking distance. 
2. Method of detecting the course of currents in electrical discharges. 
In varying the experiment of the pith-ball, taper-flame, and float- 
wheel, the following method was found convenient, and in some res- 
pects highly satisfactory, as it served to show the influence of magni- 
. tude in the balls on the apparent direction of the currents of electri- 
