The Basics

• Multimeters can function as ammeters, where they can measure current, or as voltmeters, where they are used to measure voltage. Another typical role is that of ohmmeters, where they measure resistance. They have a knob or button which allows them to switch between the various functions. Higher-end ones are pricier because they have additional capabilities and are more precise. Precision is required when measuring components that may be used on NASA space shuttles, for example.
  
  Multimeters range in size from small hand-held devices to large tabletop ones. The tabletop ones usually require AC voltages to operate. Hand-held ones need their batteries periodically replaced. Both large and small ones need to have their fuses changed occasionally for the purposes of maintenance.
  
  

Voltage Measurements

• Digital multimeters have two voltage settings--AC and DC. It is important to pick the appropriate setting, lest the readings be inaccurate. Voltmeters should not have any current passing through them, and so the digital multimeter must be placed in parallel with the circuit. Their leads must be placed on each side of the component of interest. Polarity must be observed and so that their positive and negative leads must be connected to the positive and negative sides of the circuit, respectively.
  
  

Current Measurements

• Digital multimeters functioning as ammeters must have current passing through them to measure it, and so must be placed in series inside a circuit. Polarity must be observed here as well. Once again, it is important to choose the proper AC or DC setting.
  
  

Resistance Measurements

• These must be performed outside the circuit or with the power to the circuit off. The digital multimeter must be placed on its resistance setting, and a probe must be placed on each side of the component that is to be measured.
  
  Resistance measurements are used to verify the accuracy of the element or device in question. They may also be used to find the amount if one has an element with an unknown value. This may be done by choosing the lowest setting and then switching to higher ones until a reading is obtained.
  
  

Other Measurements

• Many multimeters have what is known as a continuity setting. A continuity setting is used to test whether a circuit is complete. A complete circuit has current flowing through it, while an open circuit has a break inside of it. This may be due to a damaged component, for example.
  
  Advanced multimeters may also have settings to measure diodes, inductance and capacitance. Some are capable of displaying transistor properties, such as the gain or amplification. Some even have the ability to perform frequency and logic measurements. These are indicated in the user's manual or by symbols on the instrument itself.