Uniform Field Spark Gaps
suggested the use of uniform field spark gaps for the measurements of a.c., d.c
and impulse voltages. These gaps provide accuracy to within 0.2% for a.c.
voltage measurements an appreciable improvement as compared with the equivalent
sphere gap arrangement.
shows a half-contour of one electrode having plane sparking surfaces with edges
of gradually increasing curvature
portion AB is flat, the total diameter of the flat portion being greater than
the maximum spacing between the electrodes. The portion BC consists of a sine
curve based on the axes OB and OC and given by XY = CO sin (BX/BO . π/2). CD is
an arc of a circle with centre at O.Bruce showed that the breakdown voltage V
of a gap of length S cms in air at 20°C and 760 mmHg. Pressure is within 0.2
per cent of the value given by the empirical relation.
This is a
great advantage, that is, if the spacing between the spheres for breakdown is
known the breakdown voltage can be calculated.
advantages of uniform field spark gaps are
influence of nearby earthed objects
the disadvantages are
accurate mechanical finish of the electrode is required.
parallel alignment of the two electrodes.
of dust brings in erratic breakdown of the gap.
much more serious in these gaps as compared with sphere gaps as the highly
stressed electrode areas become much larger. Therefore, a uniform field gap is
normally not used for voltage measurements.