Analog ammeters, also known as current meters, are metered instruments that measure current flow in amperes. Current levels are displayed on a dial, usually with a moving pointer or needle made of a soft iron. Analog ammeters provide information about current draw and current continuity in order to help users troubleshoot erratic loads and trends. They have both positive and negative leads and feature extremely low internal resistance.

They are required to measure the current in a circuit and are therefore connected in series with the components carrying the current. High current flow may indicate a short circuit, unintentional ground, or defective component. Low current flow may indicate high resistance or poor current flow within the circuit. If the ammeter resistance is not significantly smaller than the load resistance, the load current can be substantially altered by the inclusion of the ammeter in the circuit.

For higher range ammeters, a low resistance made up of manganin (low temperature coefficient of resistance) is connected in parallel to the moving coil and the instrument may be calibrated to read directly to the total current. This type of ammeter is called a shunt. The shunt method is typically used for higher currents as the coil of the movements can only handle small currents directly.

Most current meters do not require power to take a measurement. The current flows through the meter and causes the meter to move. Many of the same movement types found in the voltmeters selection guide can be used in ammeters.

The advantages of analog ammeters is that they are suitable for use in AC and DC circuits, made of simple parts, and low cost compared to a moving coil instrument. The instrument is robust due to simple construction and the torque/weight ratio is high, leading to less frictional error. Disadvantages include: errors due to variation in temperature and frequency, relatively low values of magnetizing force cause by stray fields, and discrepancies caused by friction.

Specifications 

Analog ammeters can measure levels of alternating current (AC) and direct current (DC). Some devices that measure AC current also measure root mean square (RMS) power, which is the square root of the time average of the square of the instantaneous power. Many analog ammeters include a current sensor built into the meter or that clamps around the wire.

Different types of analog ammeters can measure different ranges of AC current, DC current, and AC current frequency.

Sensitivity

Ammeter sensitivity is determined by the amount of current required by the meter coil to produce full-scale deflection of the pointer. The smaller the amount of current required producing this deflection, the greater the sensitivity of the meter. A meter movement that requires only 100 microamperes for full-scale deflection has a greater sensitivity than a meter movement that requires 1 mA for the same deflection.

Ranges

For analog ammeters, the ampere-turns necessary to produce full-scale deflection are constant. The range can be altered by providing a shunt coil with the moving coil.  

Features

Some devices can test the operation of batteries or diodes. Others include programming or components designed to counteract known errors caused by temperature changes. Mirrored scales make it easier to read analog ammeters to a given accuracy by enabling operators to avoid parallax errors. Some devices are handheld and portable, while others are designed for benchtop or shop floor use.

Standards

Analog ammeters carry a maximum rating and are used in a variety of applications and industries. QPL-6752 is a list of qualified products under performance specification MIL-A-6752 for ammeters, voltmeters, and loadmeters, and BS 89-2 documents direct acting indicating analog electrical measuring instruments.  

Resources

Measuring Instruments

Moving Iron Instruments - Voltmeter and Ammeter

Image credits:

Weschler Instruments |  NPTEL | Weschler Instruments| Cole-Parmer