Error Analysis of Internal Connection Method and External Connection Method of Ammeter
If the resistance value of the resistance R to be measured is small, the resistance value measured by the external connection method is more accurate and the error is small; if the resistance value of the resistance R to be measured is large, the resistance value measured by the internal connection method is more accurate.
(1) When using the external connection method
The number displayed in the ammeter should be the sum of the current IR flowing through R and the current IV passing through the voltmeter, that is, I=IR+IV, then R=U/I=U/(IR+IV)
The cause of the error is that the shunt current IV of the voltmeter is added to the current I, and as a result, the measured value of R is smaller than the actual value. However, the internal resistance RV of the general volt charter is relatively large, about several thousand ohms. If the resistance R to be measured is much smaller than the internal resistance RV of the voltmeter, that is, R< The relative error is: The greater the internal resistance of the voltmeter, the smaller the relative error. (2) When the internal connection method is used The voltage representation number U is equal to the sum of the voltage UR at both ends of R and the voltage UA at both ends of the ammeter U=UR+UA, that is, then The cause of the error is that the voltmeter divider voltage UA is added to the voltage U, and as a result, the measured R value is larger than the actual value. If the resistance value of the resistance R to be measured is much larger than the internal resistance RA of the voltmeter, that is, R>>RA, according to the nature of the voltage division, UA can be ignored. It is considered that U=UR, and the measured results are comparative. The relative error is: It can be seen that the smaller the internal resistance of the ammeter, the smaller the relative error. The resistance value of the resistance to be tested is relative to the internal resistance of the instrument used. An ammeter with a large range has a small internal resistance, and a voltmeter with a small range has a large internal resistance; a voltmeter with a large range has a large internal resistance, and a voltmeter with a small range has a small internal resistance. However, the amperage or volts represented by each small division of the ammeter with a large range is relatively large, and it is impossible to read more accurate values for small currents and voltages.
