The ampermeter and the voltmeter are the devices intended for measurement of electric current. But current parameters which are measured by means of these physical devices, different.
Tells its name about purpose of each of these devices. One of them comes from the word "ampere", so is called unit of measure of force of electric current, and it is measured by means of the ampermeter. The volt is a unit of measure of electromotive force and voltage, therefore, the voltmeter also is intended for measurement of these parameters.
The ampermeter and the voltmeter have a similar design. These are magnetoelectric devices, the measured sizes are brought in them to a winding, and its magnetic flux affects a permanent magnet. The mobile magnet is connected with an arrow or the measuring coil. At the ampermeter it is connected to the shunt which is installed either in the device, or out of it, and at the voltmeter the measuring chain is included in the place where perform measurement.
The difference in their principle of action is explained by different purpose of devices. In order that the device could measure current, internal resistance has to be minimum, and at the ampermeter it such is. High resistance could change the current which the ampermeter also measures in an electrical circuit, in this case the result would be distorted. The ampermeter with a zero resistance would be ideal, but it is impossible, and devices possess different degree of sensitivity. Depending on it their scales are graduated in amperes, kiloamper or milliamperes.
With measurement electric tension the situation is exactly the opposite – change of current, more precisely, decrease it is necessary, only in such a way it is possible to avoid change of tension which the voltmeter has to measure. In ideal option the internal resistance of the voltmeter has to be infinite, but in reality it is unattainable, and still it is greatest possible. Than internal resistance is higher, especially voltage measurement will be exact. The ampermeter and the voltmeter are differently connected to an electrical circuit for measurement. The ampermeter is connected consistently with that its site where it is necessary to measure current. The voltmeter is connected parallel to that site of an electrical circuit on which measure tension. It is impossible to connect the ampermeter directly to the power supply or to both conclusions of electric current at all as it is done with the voltmeter. Similar connection can lead to short circuit and the output of the device out of operation.