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Avometer is a British trademark for a line of multimeters and electrical measuring instruments; the brand is now owned by the Megger Group Limited. The first Avometer was made by the Automatic Coil Winder and Electrical Equipment Co. in 1923, and measured direct voltage, direct current and resistance. 〔GB Patent 200977〕 Possibly the best known multimeter of the range was the Model 8, which was produced in various versions from the May 1951 until 2008; the last version was the Mark 7. The multimeter is often called simply an AVO, because the company logo carries the first letters of 'amps', 'volts' and 'ohms'. The design concept is due to the Post Office engineer Donald Macadie, who at the time of the introduction of the original Avometer in 1923 was a senior officer in the Post Office Factories Department in London. Mr. MacAdie (1871 - 1955) was born in the county of Caithness in northern Scotland. (The local pronunciation of MacAdie rhymes with Caddy). ==Technical features== The original Avometer was designed to measure direct current (3 ranges, 0.12, 1.2 & 12A), direct voltage (3 ranges, 12, 120 & 600V) and resistance (single range, 0 - 10,000 ohms, 225 ohms mid-scale). All ranges could be selected by a single rotary switch which set both the function and the range value. A second switch brought a rheostat into circuit in series with the instrument and could be used to control the current through a device under test and the meter. The movement drew 12mA for full-scale deflection and used a "universal shunt" permanently in parallel with the movement which increased the input terminal full-scale current to 16.6mA, corresponding to 60 ohms per volt. It had a knife edge pointer and an anti-parallax mirror. Additional patents were taken out in Czechoslovakia (1923),〔Czech Patent CA236252〕 Austria,〔Austrian Patent AT98613B〕 France,〔French Patent FR566295〕 Germany,〔German Patent DE405863〕 and Switzerland 〔Swiss Patent CH107655〕 (1924). A US patent 〔US Patent US1593024〕 followed in 1926 The case of the original Avometer was a comb-jointed oak box with an ebonite lower front panel. The upper part of the front panel was cast aluminium. After around three years production, the volume of sales was sufficient to justify a redesign of the instrument, now with a movement whose full-scale current was 6mA. The redesigned meter had 13 ranges and was constructed on a one piece phenolic moulding with the characteristic "kidney" shaped window. The back case was a deep drawn aluminium can on the back of which was a summary of the operating instructions, a feature of all future Avometers. The movement was originally protected by a short length of wire, selected to act as a fuse, soldered to supports on the back of the movement. Later versions had a calibrated, screw-in, fuse on the front panel. After copper oxide instrument rectifiers became available in the late 1920s, a 20-range "Universal" version of the Avometer was introduced in 1931 having both direct and alternating voltage current ranges.〔L. O. Grondahl and P. H. Geiger, A New Electronic Rectifier, A. 1. E. E. 46, 357-366 (1927).〕〔GB Patent GB404015〕 Unlike many similar multimeter designs, all Universal Avometers, with the exceptional of the short-lived "High Resistance (HR) Avometer" (c. 1948 - 1951), could measure up to either 10A or 12A (AC) depending on the model. From 1933, the number of available voltage and current ranges in Universal Avometers was doubled by incorporating a dual sensitivity movement circuit.〔GB Patent GB423199〕 The higher sensitivity was selected by a push button switch marked ÷2 (Divide by two) signifying that the pointer indication should be halved. For the Model 8, this feature was not used but the push button was retained for reversing the direction of deflection of the moving coil. A design feature of Avometers was simplicity of use and towards this end, all measurements could generally be made using only two input terminals. However, the Avometer HR had additional 2500V (AC) and (DC) ranges which used the corresponding 1000V ranges, and were connected through two additional terminals at the top corners of the front panel. This feature was continued in the Model 8 and, with an increase to 3000V to match their 1 - 3 - 10 ranges sequence, in the Model 9, Marks II and IV and the Model 8 Mark V. The 3000V ranges were deleted in the Model 8 Marks 6 and 7 due to concerns for compliance with contemporary safety standards. This also led to a significant cost saving by eliminating the high voltage multiplier resistors. As an ohmmeter the Model 8 Mark II measures from 1 Ω up to 20 MΩ in three ranges. The instrument has an accuracy of ±1% of FSD on DC current ranges, ±2% of FSD on DC voltage ranges, ±2.25% of FSD on all AC ranges and ±5% of reading (at ''centre scale'' only) on resistance ranges.〔http://frank.pocnet.net/instruments/AVO/HR/AvoMeters/Avometer8MkI&IIservice.pdf AVO Model 8 Service Manual.〕 Its maximum current draw of 50 μA at full-scale deflection (corresponding to 20,000 ohms per volt) is sufficient in most cases to reduce voltage measurement error due to circuit loading by the meter to an acceptable level. The Avometer design incorporates an electrical interlock which prevents AC & DC ranges being selected simultaneously.〔GB Patent GB404015 Claim 6〕 For example, none of the DC ranges, current or voltage, can be connected unless the AC switch is set to its "DC" position. On a Model 8, this is the position with the AC switch arrow vertical. Similarly, to use the AC ranges, the DC switch must be set to its "AC" position. With the DC switch set to its "AC" position and the AC switch set to "DC", no current can flow through the instrument. However whenever any moving coil instrument is likely to be subjected to heavy shock in transit, it is good practice to damp the movement by short circuiting the moving coil using a heavy gauge wire connected across the terminals. On earlier Avometers, this may be done by short-circuiting the input terminals and selecting the most sensitive direct current range. The Model 8 Mark V, 6 & 7 were provided with an "OFF" position on the DC switch which both disconnected the meter's terminals and short-circuited the moving coil. Avometers designed from 1936 onwards were fitted with an overload cut-out operated by the moving coil frame hitting either forward or reverse sprung end stops.〔GB Patent GB464867〕 The Model 7 was the first type to use the end stop cut-out and it also featured an acceleration trip which, in the event of heavy overloads, could open the cut-out before the pointer had reached two-thirds of full scale. The acceleration cut-out was not however used in the Model 8. From the Mark III version, the Model 8 had further protection by a fuse on its resistance ranges and fuse protection was provided on all ranges of the Model 8 Marks 6 & 7. AVO multimeters were almost ubiquitous in British manufacturing and service industry, research and development and higher and further education. They were also widely used by utilities, government agencies and the British armed forces. A number of special versions were produced to British Admiralty and Air Ministry specifications and for other customers. The Model 8 Marks V, 6 & 7 were designed to meet a NATO specification and were standard issue to NATO services. Many commercial and military service manuals specified that values for measurements of current or voltage had been made with a Model 7 or Model 8 Avometer. Advertisements of the late 1930s compared the utility of the Avometer to the slide rule. Even nowadays it can still be found in regular use. The earlier versions of models 7, 8 and 9 had a design flaw which resulted in many instuments sustaining damage to the movement in transit. Users would habitually 'switch off' the instrument by setting the AC switch to 'DC' and the DC switch to 'AC'. With the switches at these settings, the movement is completely undamped.〔Model 7, 8 and 9 schematic diagrams.〕 The operating manuals for the affected instruments did contain a note that they should not be switched to 'AC' and 'DC' (or the blank position either side of the 'AC' and 'DC') though failed to explain why.〔Avometer user manual for Avo 8 Mk 2.〕 The problem was solved on later instruments by providing the DC switch with an 'OFF' position (see illustration above). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Avometer」の詳細全文を読む スポンサード リンク
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