Spark, Duration of Electric.
Wheatstone determined the duration of the spark given by a Leyden jar as
1/24000 second. Feddersen by interposing a tube of water 9 millimeters
(.36 inch) long in its path found that it lasted 14/10000 second, and
with one 180 millimeters (7.2 inches) long, 188/10000 second. Lucas and
Cazin for a 5 millimeter (.2 inch) spark, with different numbers of
Leyden jars, found the following:
Number of jars. Duration of Spark.
2 .000026 second
4 .000041 "
6 .000045 "
8 .000047 "
The duration increases with the striking distance, and is independent of
the diameter of the balls between which it is produced.
Spark Gap.
The space left between the ends of an electric resonator (see Resonator,
Electric) across which the spark springs. Its size may be adjustable by
a screw, something like the arrangement of screw calipers.
Sparking.
In dynamo-electric machines, the production of sparks at the commutator
between the brushes and commutator sections. The sparks are often true
voltaic arcs, and in all cases are injurious if in any quantity, wearing
out the commutator and brushes.
Sparking, Line or Points of Least.
In a dynamo or electric motor the diameter of the commutator
determining, or the points on the commutator marking the position of the
brushes where the sparking is a minimum. Field magnets powerful in
proportion to the armature are a preventative cause. The direction of
the line fixes the angle of lead to be given to the brushes.
Sparking, Resistance to.
The resistance to disruptive discharge through its substance offered by
a dielectric or insulator. It does not depend on its insulating
qualities, but on its rigidity and strength.
Spark, Length of.
The length of the spark accompanying the disruptive discharge is counted
as the distance from one electrode to the other in a straight line. It
is longer for an increased potential difference between the two
electrodes. If the gas or air between the electrodes is exhausted the
length increases, until the vacuum becomes too high, when the length
begins to decrease, and for a perfect vacuum no spark however small can
be produced. The shape of the conductor which is discharged, the
material of the electrodes, and the direction of the current are all
factors affecting the length of spark producible.