Since its conception in 1923, the Avometer has maintained a
distinct lead upon all its competitors, and can today quite
rightly be termed the most popular instrument of its type in
the world, for in no other instrument can one find such a
unique combination of ranges and comprehensive automatic
overload protection, in addition to a high degree of
accuracy, reliability and simplicity of use.
Much time and thought is continually devoted by our design
department to the improvement of our products and it is for
the Electronic Radio and Television Engineer that this new
instrument has been primarily produced. The Model 9 Avometer
Mk. II has the high d.c. voltage sensitivity of the Model 8
Avometer, but is easier to read due to improved scaleplate
design. A further useful feature which has been retained is
the push-button change-over switch which enables the
direction of the current through the moving coil to be
reversed, thus obviating the necessity of changing leads
when working with d.c. voltages and currents which may be
either positive or negative in respect to a basic test
position. The excellent qualities of previous models
including the AVO automatic cut-out have been retained, and
we have great confidence that given a reasonable amount of
care and attention, not forgetting the removal of exhausted
batteries, this instrument will give lasting satisfaction.
The meter is extremely simple to use, range selection in
general being accomplished by means of two switch knobs. All
tests, except those on the 3kV ranges, make use of the pair
of terminals at the base of the instrument.
A clearly marked 5-in. scale has uniformly divided
graduations to match 100 and 300 scale markings, and in
addition there is an ohms scale. An anti-parallax mirror
permits readings of the knife edge pointer to be made with
An important and entirely new feature of the instrument is
that all range switches, controls and terminals are
identified with graphical symbols which in general follow
the recommendations of the International Electrotechnical
The meter is supplied complete with a pair of special rubber
covered leads which are intended for attachment to the
Avometer by means of its captive terminals. The remote ends
of the leads are fitted with spring clips, which may be
interchanged with the AVO Long Reach Safety Clips Mk. II
supplied with the instrument which have been introduced to
enable connections for test purposes to be made at what are
normally inaccessible points on a chassis.
Limits of Accuracy
Generally speaking, the highest percentage accuracy on
current and voltage ranges is obtainable at the upper end of
the scale, but on resistance ranges it is better towards the
centre of the scale. In the case of voltage measurements,
which are more frequently taken than
chose of current, successive ranges have been closely chosen
to obviate the need for taking readings on very small
The instrument will produce its highest accuracy when used
face upwards, in which position it has been calibrated.
D.C. Voltage. 2% of indication between full-scale and
half-scale deflection. Below half-scale deflection, 1% of
the full-scale value
D.C. Current. 1 % of full-scale value over effective range.
A.C. Voltage. 2-25% of full-scale value over effective range
A.C. Current. 2-25% of full-scale value over effective range.
‘Effective range’ is in accordance with British Standard
Specification 89/1954 as follows,
D.C.—from 0-1 of f.s.d. to f.s.d.
A.C.—from 0-25 of f.s.d. to f.s.d.
It will be noted that with the exception of the d.c. voltage
ranges, the instrument meets the requirements laid down in
Section 6 of the British Standard Specification 89/1954 for
5-in. (127mm.) scale-length Industrial Portable Instruments.
In practice, the Model 9 Mk. II is well within the above
limits, due to the great care taken in the manufacture of
its various components, and to the fine initial calibration.
Inasmuch as rectifier moving coil instruments give readings
on ‘a.c.’ proportional to the mean and not the r.m.s. value
of the wave form with which they are presented, they depend
for their accuracy not only upon their initial calibration,
but also upon the maintenance of a sinusoidal wave form.
Since the form factor (r.m.s. value divided by mean value)
of a sine wave is 1-11, this has been taken into account in
calibrating the meter which does, therefore, indicate r.m.s.
values on the assumption that the normal sine wave will be
encountered. Generally speaking, considerable wave form
distortion can occur without appreciably affecting the form
factor and resulting accuracy of measurement, but the user
should recognise the possibility of some error when using
distorted wave forms, squarish wave shapes producing high
readings, and peaky ones, low readings.