How to Measure Voltage & Current

Voltage is the electromotive force that moves electrons through conductive objects. We know from Ohm's law (V = I × R) that the voltage across an object is equal to the current through that object times the resistance of that object. If we know any two of the three variables (voltage, resistance, and current), we can calculate the other. To measure voltage we must first set the voltmeter to a scale that is at least a little higher than the highest voltage you are using. For example, if you are using a 6 volt battery then set the scale to the 10 or 20 volt range. If you are using direct current (DC), like a battery, then make sure the voltmeter is set for DC. Voltmeters will normally have a red (positive) lead and a black (negative) lead, as shown schematically in Figure 1.

Figure 1. Simplified digital voltmeter diagram.

If you are using alternating current (AC), such as exists in the outlets of your house, then use the AC setting. In this case it does not matter which lead you use since alternating current does not have polarity.

In Figure 2 we have a sample circuit that contains a DC voltage source called V_in and some circuit elements labeled 1 through 4.

Figure 2. Circuit diagram with voltage source and 4 circuit elements.

Place the voltmeter leads across the object in the circuit you wish to measure the voltage across being careful to put the positive lead (red) on the side closest to the positive terminal of your voltage source or battery and the negative lead (black) on the side closest to the negative terminal of your voltage source or battery.

Figure 3. Measuring the voltage across circuit element 4.

If you are measuring AC voltage, the polarity of the leads does not matter. You can also place the voltmeter directly on the battery or voltage source to determine the voltage of that source. Now read the voltage off the meter. If the value is very low you can lower the range on the meter until you get a satisfactory reading. You should use the lowest range possible without going over the range—this could damage the meter.

Current is the flow of electrons through a conductor per unit of time. The unit of current is the ampere (A), sometimes referred to as "amps" for short. This unit is equal to one coulomb (= 1.6021891 × 10¹⁹ electrons) per second. This is a relatively large amount of current, so we often use the unit of milliamperes (mA) which is one thousandth of an ampere. In any circuit the current flowing through elements in series is the same by definition, since there is only one path for the current to flow. To understand this better see Kirchhoff's voltage and current laws (see Sources, below, for an online reference).

Place the meter in the current position. Use either DC or AC, depending on the type of voltage source you are using. Use the highest current range to start as you can always lower the range if the reading is too low. Break the connection with the component that you want to measure the current in and place the meter in series with that component. If you are using a DC voltage source place the red lead closest to the positive lead of the voltage source and the black lead toward the negative side of the voltage source. Then read the current from the meter. If the value is very low you can lower the range on the meter until you get a satisfactory reading. You should use the lowest range possible without going over the range as this could damage the meter. In Figure 4 we are going to measure the current flowing through element 4.

Figure 4. Measuring the current through circuit element 4.

Sources

• Written by Kurt Taylor, .
• Illustrations kindly provided by E. J. Mastascusa, Professor of Electrical Engineering, Bucknell University:
  Mastascusa, E.J., 2006. "An Introduction to Electrical Measurements," Department of Electrical Engineering, Bucknell University [accessed March 14, 2006] http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Measurements/MeasIntro.htm.
  
  
• An online source for information on Kirchhoff's Laws is:
  Burley, I. et al., date unknown, "Kirchhoff's Laws," Introductory Physics Notes, University of Winnipeg [accessed March 14, 2006] http://theory.uwinnipeg.ca/physics/curr/node8.html.