Current Measurement; Resistance Measurement - Megger AVOMETER 8 Mk7 Guia Del Usuario

consumes 1 mA at full scale deflection. The 3 V
a.c. range consumes 10 mA at full scale deflection
(100 Ω/V). The meter maintains a high degree of
accuracy for audio frequency tests up to 15 kHz
on ranges up to 300 V a.c.
All current consuming voltmeters, however
sensitive, draw current to varying degrees from
the circuit under test, thus causing the voltage to
fall at the point of measurement. Owing to the
high sensitivity of the Model 8mk7 d.c. ranges,
this effect is unlikely to be of importance except
in a very few instances. It might affect the
measurement of e.h.t. voltage on a television set
or the tapping on a potential 'divider', where the
resistances are comparable with the resistance of
the instrument on the range in use. It is generally
possible to use an instrument on a higher range
than absolutely necessary where the higher
instrument resistance causes less disturbance. At
the same time adequate pointer deflection for
reasonable accuracy should be attained.
When it is essential to obtain an accurate
indication of the voltage developed across a high
value resistor it is sometimes preferable to insert
the instrument in series with it and to measure
the current flowing. The reading in milliamps,
multiplied by the value of the resistance in
10
thousands of ohms, will give the developed
voltage.
Great care must be exercised when making
connections to a live circuit and the procedure
should be entirely avoided if possible.

CURRENT MEASUREMENT

To measure current the instrument should be
switched to "AC ~" or "DC . . . " as appropriate
and a suitable a.c. or d.c. range set by the other
switch. Connect the instrument in series with the
circuit under test, do this only when that circuit is
switched off.
The voltage drop at the meter terminals is
approx. 710 mV on the 10 A d.c. range at full
load reducing to 100 mV on the 50 µA d.c. range
at full load. In the case of a.c. it is 390 mV at 10
A and 480 mV at 10 mA. Standard meter leads
have a resistance of 0,02 ohm per pair.

RESISTANCE MEASUREMENT

There are three self-contained ranges covering
resistances from 1 Ω to 20 mΩ.
On resistance ranges the meter must have, in
addition to the normal instrument mechanical
zero, a resistance zero corresponding to the full
scale deflection of the meter. Before carrying out