Why to expand the range of DC ammeter need parallel resistance shunt on the meter, why to expand the range of DC voltmeter need series resistance on the meter

Why to expand the range of DC ammeter need parallel resistance shunt on the meter, why to expand the range of DC voltmeter need series resistance on the meter

This problem is just a simple problem of shunt resistance and shunt series resistance
When applied to the ammeter and voltmeter, the key is not to say that the display increases, but the actual measurement is only part of the output. For example, when measuring the current, the full range of the meter is 10a, and the measured value is also 10A. Parallel a resistance with the same resistance as the ammeter, the measured value becomes 5A. If you are a high school student, have a good look at the textbook

There is an ammeter g. when it is refitted into a voltage meter with a range of 3V, a 1000 Ω resistor should be connected in series. If it is refitted into a voltage meter with a range of 8V
When the ammeter is in series with a 3500 Ω resistor, what is the internal resistance Rg of the ammeter? What is the full current Ig?

In the voltmeter, G is connected in series with the resistor. The range is the total voltage in series = full bias voltage + voltage at both ends of the resistor
Let the full bias current of the voltmeter be Ig
When the measuring range is 3V: UG + U = u total
RgIg+1000Ig=3.
When measuring range is 8V: UG + U '= u total'
RgIg+3500Ig=8.
The solution is: RG = 500 Ω

What is the purpose of sliding the rheostat slide in the experiment of "measuring resistance with ammeter and voltmeter"

Sliding rheostat has two functions: protection and regulation
In the experiment of "measuring resistance with ammeter and voltmeter", the purpose of sliding rheostat slide is to adjust the voltage at both ends of the resistance to be measured and the current to provide the resistance to be measured, so as to obtain multiple groups of corresponding voltage and current measurement values. It is convenient for further analysis and demonstration of multiple resistance values, and obtain reasonable results (conclusions)