A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit. The voltage can be measured by allowing it to pass a current through a resistance; therefore, a voltmeter can be seen as a very high resistance ammeter. One of the design objectives of the instrument is to disturb the circuit as little as possible and hence the instrument should draw a minimum of electric current to operate. This is achieved by using a sensitive ammeter or microammeter in series with a high resistance. Note that voltmeters built on this principle show varying input resistance as the instrument is switched through its measuring range. The meter will generally specify a number of Ohms/Volt on the faceplate. Multiplying this number by the voltage range the meter is set to gives the input resistance of the instrument. The moving coil galvanometer is one example of this type of voltmeter. It employs a small coil of fine wire suspended in a strong magnetic field. When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current that is flowing through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.
Volume Compensators are used for a vapor canister which includes a partition adapted to be disposed in an interior chamber of the vapor canister and having a single opening extending. The volume compensator assembly also includes a grid pattern disposed on the partition about the opening to guide airflow to the opening. This assembly includes a spring connected to the partition and contacting the vapor canister to move the partition for compensation of a volume of a bed of vapor adsorbing material in the vapor canister.