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. General Description 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 great precision. 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 Commission (I.E.C.). 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 deflections. 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 (25-2000 c/s). 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.