CHAPTER 19
NOTES ON HIGH VOLTAG—E TECHNIQUE
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(See
chapters on X-rays and the Acceleration of Ions.)
,
THE MEASUREMENT or HIGH VOLTAGE
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19—î.
Sphere Gaps.—When the separation between two
charged spheres is graduallyieduc‘ed, a critical distance between
their closest surfaces will be found where the air will break down
and a spark occur. This distance or gap depends on the voltage
difference between the spheres. Values Will be found in table 3,
together With the conditions under which the measurements are to
be made.
Foreign bodies, Whether insulators or conductors, in
the neighborhood of the gap Will distort the electro-static eld and
alter the values.
For accurate work the spheres should befreshly—
polished. When a spark takes place in a sphere gap, sparks may
occur in distant parts of the room.
If these occur between current
carrying wires, an arc may form, burning out motors, lamps, etc.
Use fuses or circuit breakers in all power lines.
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19—2.
Resistance Types.—Another method of determining
high voltages consists in measuring the potential drop across a
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portion of a very high resistance con… D
nected across
line.
As an alterna—
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.
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_
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—\R
tive method,
a small current
meter may
—
.
be placed in series With a known, high
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D
resistance across the line and Ohm’slaw
"i,!
.
‘
=
applied. Such resistances are available
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‘
—— '
on the market, sometimes With corona
‘
'… D
shields.1
“Metallized” resistors have a
_
of high VŒY
thin metallic .hÏ} °F,
an
résistances
at
rod_1n proteCtwe caSing.
num: —_
,
…
volt/ages,
,
'
_
'
‘each other may
in gure
19—1 In this gure, a large number (800)‘ofÿægistäæ each “of
10megohms, are placed end ,to end in, contact With
'
each other in