11-01-2013, 11:06 PM
Back in the 30's, meters with a mechanical pointer and analog scale were used to measure voltages instead of more modern VTVM's and DVM's.
These analog meters used a milliammeter movement with 1 mA full scale reading, in series with a resistor to measure voltage. Because it took 1000 ohms of total series resistance to make the meter read 1 volt full scale, they were called 1000 ohms per volt meters.
On higher voltage ranges, the resistance increased proportionally. For example, on the 10 volt range the series resistance was 10,000 ohms and on the 100 volt range it would be 100,000. Note that the ratio always remained 1000 ohms per volt as a constant.
The net result of all this is that an analog 1000 ohms per volt meter will place a greater load on a circuit than will a DVM which has a constant 10 megohm input resistance. So on the 100 volt scale, the analog meter will load the circuit with 100K to ground while the DVM will load it with a negligible 10 megohms. Therefore in high resistance circuits like the plate of a first audio stage, the analog meter will read a lower voltage than will a DVM. Measuring directly across the power supply caps, both meters will read the same because the difference in loading has no effect.
These analog meters used a milliammeter movement with 1 mA full scale reading, in series with a resistor to measure voltage. Because it took 1000 ohms of total series resistance to make the meter read 1 volt full scale, they were called 1000 ohms per volt meters.
On higher voltage ranges, the resistance increased proportionally. For example, on the 10 volt range the series resistance was 10,000 ohms and on the 100 volt range it would be 100,000. Note that the ratio always remained 1000 ohms per volt as a constant.
The net result of all this is that an analog 1000 ohms per volt meter will place a greater load on a circuit than will a DVM which has a constant 10 megohm input resistance. So on the 100 volt scale, the analog meter will load the circuit with 100K to ground while the DVM will load it with a negligible 10 megohms. Therefore in high resistance circuits like the plate of a first audio stage, the analog meter will read a lower voltage than will a DVM. Measuring directly across the power supply caps, both meters will read the same because the difference in loading has no effect.
