Whether digital or analog, a voltmeter is subject to an electrical theory known as Ohm’s Law. Ohm’s Law describes the interrelationships in an electrical circuit. The mathematical formula depicting Ohm’s Law is given in a variety of ways. One is P=IE. In this equation P, or Power, is equal to I (current) times E (voltage). This can be further simplified using resistance, voltage and current. In this case current would be the result of voltage divided by resistance. Or, resistance is the result of voltage divided by current. Analog and digital meters can be used to measure these values in a circuit. Below you will find a brief description of how an analog meter measures voltage.
An analog voltmeter gives readings by deflecting a pointer along an analog scale. On most analog meters, there is a rotating switch which chooses between resistance scales across the meter face. There will normally be a 20 Volt scale, a 200 Volt scale, a 2000 Volt scale and on some meters, a 20,000 Volt scale. These scales will correspond to the setting of the rotary switch. These settings will be separate for alternating and direct current.
Taking Your Measurement
In order to take a voltage reading with an analog voltmeter, you will need to know approximately how much voltage should be present in the circuit at the point of measurement and select the proper scale. In other words, if you expect to find 12 volts at the point of measurement, and that voltage is direct current, then you will need to set the meter to the 20 Volt DC scale. You will then place your positive lead on a ground point and the red lead at the actual point of measurement. If measuring a voltage drop across a device or resistor, then your black lead will go on the negative side of the load and the red will go on the positive side. Remember, since an analog meter is not capable of indicating a negative voltage, you must pay careful attention to circuit and test lead polarity in order to receive accurate measurements.
How It Works
An analog voltmeter works by passing a current through a coil that is suspended between two permanent magnets. This coil of wire is known as a moving coil since it moves in relation to the permanent magnets when a voltage is applied. When a voltage is applied and the voltage scale is chosen, a resistor of known value is placed in series with the measurement leads. This way Ohm’s Law can be applied. The applied voltage through the coil creates a magnetic field which acts against the permanent magnets that the pointer pivot is placed between. This magnetic field causes a corresponding deflection of the pointer. This pointer deflection will be in direct proportion to the amount of voltage being applied to the moving coil wrapping the pointer pivot. Once pointer oscillation has stopped, accurate readings can be taken.
An analog voltmeter is an extremely accurate and very sensitive piece of electrical test equipment. In the preceding paragraphs, you have been given a brief description of they work.