DESIGN
FEATURES
DESIGN
AND
CONSTRUCTION
The instrument comprises a moulded panel on which are
mounted the
whole of the switching apparatus, the move-
ment and other components. A carrying strap is fixed
to
moulded lugs on the rear
case.
The front panel is fitted
into this
case
with a dust proof joint. The switches
are
of
generous and robust design, the contacts being arranged
to
'make' before 'break' on adjacent ranges; a feature which
provides a factor of safety in
use.
When the instrument is set for operation on de the moving
coil is associated
with
a universal shunt and series
mu
lti-
pliers, whilst
on
ac,
diodes and a transformer
are
al
so
intro-
duced.
RANGE CONTROLS
The left-hand knob provides all the de current and voltage
ranges (except 3000V) and the right-hand knob the ac
ranges (excep't
3000V) and also the resistance ranges.
These knobs
are
electrically interlocked
so
that readings
can
only be made after
ac
or de measurement and range
has
been selected. Resistance tests require the left-hand
switch
to
be set
ton
and the right-hand one to the desired
range.
The ranges are marked on the
panel and arrow heads
in-
dicate the actual range selected. The 3000V ac and de
ranges
are
available at the
two
3000V terminals.
10
Wide coverage in resistance
has
been achieved by having
a
fundamental range
as
marked on the scale, together
with
ranges
of
x 100 and x 1
Ok
to
supplement
it
.
In
addition a 200 megohm range marked INS is available,
using an external
de
voltage source (See
Page
7)
.
THE
MOVEMENT
The meter movement in the Model 8 Mark 5 is a robust
AVO
centre pole movement type ACP1, fitted
with
spring
mounted
jewelled bearings and impregnated coil. The
meter
has
a full
sca
le deflection
of
37.5v.A. A knife edge
pointer
enables very fine readings to
be
taken, whilst the
whole movement
is
balanced and damp
ed
so that the
pointer
quickly comes to rest.
SCALING
The scale plate
has
three main sets
of
markings.
One
is for
resistance measurement and is marked
0 to 2000 ohms, the
second is for current and
voltage (both ac and de) and is
marked
0
to
10
whilst the third scale calibrated 0
to
3
has
60 divisions and is used for current and voltage
measurements.
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