l,.
19.
PROBLEMS OF INSULATION
445
In the modied form 5 of the generating voltmeter shown in
gure 19—4 (where there is no commutator), the xed and insulated
quadrants (z), are alternately shielded and exposed to the electrostatic eld by the rotor plates (Â2). The alternating current
_
which is then generated ows through the resistance R, creating
a potential drop which is amplied, rectied and then observ£‘ed
on the meter (M).
By the use of several values of resistance at '
R, a wide range of voltages may be measured with the same meter.
19—4.
Miscellaneous—A series of condensers may be connected across the high voltage line and measurements made of the
potential drop across one of them by means of a high resistance
voltmeter.
In this
“
capacity potentiometer,” the total voltage
is calculated from the known values of the capacities; From the
_
relation Q = CV
,
it may be seen that the larger potential drops
occur across the smaller capacities.‘ Îhese
condensers must, therefore, have better insulation than those of large capacity.
High voltages may also be determined “from the step-up ratio
of the transformer
6 and by means of electrostatic repulsi0n instruments operating on the principle of the Braun voltmeter (gure
17—9).
The latter, however, must be especially constructed
to
,
‘
avoid corona or break down, as in-the attracted disc —type.7
19—5.
Indirect Measurement of Voltages.—Indirect measure—
ments of high voltage maybe madeby obsérving the magnetic
deection of ions previously accelerated by
the unknown voltage.
Also, the absorption or‘range of ions in…Various materials maybe
used to determine the voltage used.in their acceleration.
'
voltage applied to an X—ray tube-may be
ff1' gm
1nter—
'
,
cept of
?
.
‘
ruptzvecrztzcaivolz‘agalsreaChedatWh1Chwattmeær5mtheÜrcult _
.
,
begin
1She'ardfm —
'
chrtandsurfaceurfä‘äulaü‘îsalongthewms‘ThemrgYWÊÎ1
'
… il“
-