Why is it necessary to make the half deviation of pointer as big as R resistance when measuring the internal resistance of meter head with half deviation method

Why is it necessary to make the half deviation of pointer as big as R resistance when measuring the internal resistance of meter head with half deviation method

First of all, you need to make sure that the total current in the circuit is certain (there is a small change, which can be ignored here). When the pointer is half biased, the current flowing through the internal resistance of the meter head is half of that when the pointer is full biased, the other half must flow through the R resistance. The internal resistance of the meter head and the R resistance are in parallel, and the current is equal, so the internal resistance of the meter head and the R resistance must be the same
If it is not half biased, assuming that the indication is I *, then according to the equal voltage on both sides, I * times the internal resistance of the meter head = [I (full bias) - I *] times the R resistance, the internal resistance of the meter head can also be calculated, but it is not as large as the R resistance

How to measure internal resistance of meter head with ohmmeter

It is not suitable to use ohmmeter to measure the internal resistance of meter head. Because the error of ohmmeter measuring resistance is very large, it can only do preliminary measurement. The internal resistance of meter head is very small, and the purpose of measuring the internal resistance is to refit the meter, so it is not suitable to use ohmmeter to measure the internal resistance of meter head
If you have to measure it, you can find a variable resistance, measure it with a multimeter, and adjust the resistance. When the pointer points to the middle of the dial, the main value of the resistance is the resistance of the meter head

The function of the rheostat in series with the meter head in the test of measuring internal resistance with half bias method

The resistance box can read directly
When the meter is half biased, the reading of the resistance box is about equal to the internal resistance of the meter
[function of resistance and reading]
This method has principle error