58 Observations on some Ex'periments in Electricitp 



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 terra 

 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 the air, &;c. so as 

 to yield the longest possible sparks. 



A rapid succession of sparks may thus be made 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. 



The 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 lumin- 

 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, 8ic. This appearance may occur when the balls are much 

 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- 



