es previously in those
states; and the experiments of Mr. Wheatstone go to prove that it _really
begins_ at the charged surface, and from thence extends into the air (1437.
1438.) or other dielectric. According to my view, _sparks_ also originate
or are determined at one particular spot (1370.), namely, that where the
tension first rises up to the maximum degree; and when this can be
determined, as in the simultaneous use of large and small balls, in which
case the discharge begins or is determined by the latter, I would call that
discharge which passes _at once_, a positive spark, if it was at the
positive surface that the maximum intensity was first obtained; or a
negative spark, if that necessary intensity was first obtained at the
negative surface.

* * * * *

1485. An apparatus was arranged, as in fig. 129. (Plate VIII.): A and B
were brass balls of very different diameters attached to metal rods, moving
through sockets on insulating pillars, so that the distance between the
balls could be varied at pleasure. The large ball A, 2 inches in diameter,
was connected with an insulated brass conductor, which could be rendered
positive or negative directly from a cylinder machine: the small ball B,
0.25 of an inch in diameter, was connected with a discharging train (292.)
and perfectly uninsulated. The brass rods sustaining the balls were 0.2 of
an inch in thickness.

1486. When the large ball was _positive_ and inductric (1483.), negative
sparks occurred until the interval was 0.49 of an inch; then mixed brush
and spark between that and 0.51; and from 0.52 and upwards, negative brush
alone. When the large ball was made _negative_ and inductric, then positive
spark alone occurred until the interval was as great as 1.15 inches; spark
and brush from that up to 1.55; and to have the positive brush alone, it
required an interval of at least 1.65 inches.

1487. The balls A and B were now changed for each other. Then making the
small ball B inductric _positively_, the positive sparks alone continued
only up to 0.67; spark and brush occurred from 0.68 up to 0.72; and
positive brush alone from 0.74 and upwards. Rendering the small ball B
inductric and _negative_, negative sparks alone occurred up to 0.40; then
spark and brush at 0.42; whilst from 0.44 and upwards the noisy negative
brush alone took place.

1488. We thus find a great difference as the balls are rendered inductric
or inducteous; t