THREE-IN-ONE METER
HOW IT WORKS The “three-in-one” volt-ampere meter is fairly common nowadays, and perhaps some readers who use it are puzzled as to how such a small case contains apparently three instruments. ONLY ONE METER The answer is simple. There is only one meter, that is, the milliammeter, and the rest of the instrument consists of resistances arranged in such a -way as to make the pointer read volts or amperes over any required range. For instance, if a resistance of 1,000 ohms is connected in series with the milliammeter, the current in niilliamperes passing through the,resistance will be numerically equal to the potential difference applied between the free ends of the meter and the resistance. Thus the meter is now, in effect, a voltmeter. Again, if the milliammeter is shunted with a resistance, whose value is l-999th the resistance of the meter, the instrument becomes an am meter. READING IN AMPERES The fraction above is an awkward one, but it ensures that exactly 1-I,oooth of the current passes through the milliammeter consequently the current to be measured is so many milliampere times 1,000, which is the same number of amperes. The variations in range are obtained by altering the values of the resistance^.
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Bibliographic details
Sun (Auckland), Volume III, Issue 919, 12 March 1930, Page 7
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205THREE-IN-ONE METER Sun (Auckland), Volume III, Issue 919, 12 March 1930, Page 7
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