A circuit is a closed loop of wiring and components beginning at the positive side of the battery and ending back at the negative side of the battery. This loop is often closed using a common ground. Electricity only flows when a circuit is a closed loop. If the loop is open no electricity flows.
To measure voltage on a battery for example you first turn your meter to DC Voltage. Place the red probe on the positive terminal of the battery and place the black probe on the negative side of the battery or on a common ground. Your voltage will display on the screen.
Digital Multi-Meters come in two common varieties: Auto- Ranging, and Non Auto-Ranging. A non auto-ranging meter requires you to select a range of “sensitivity” for the given function you are using. For example if you want to measure resistance, you would need to select a range of up to 200 ohms, up to 20Kohms (20,000 ohms), 200Kohms, etc. An auto-ranging meter will automatically figure out the range and give you a measurement.
This can be illustrated with this math formula: Force = Pressure x Area.So if we were to push liquid at 1,000 psi into a cylinder with an end area of 2 square inches we would produce 2,000 pounds of force. If we were to enlarge the end area by using a larger diameter hydraulic cylinder with a 4 square inch end area, we would produce 4,000 pounds of force. Since the displacement of the larger cylinder will be greater, our cylinder will move slower but with greater force. Like all other forms of mechanical advantage, we trade speed for power.