A meter reads the voltage between two points. Its two leads always touch two different points.
Both leads on the same wire read zero, even on a live hot. A zero reading only means the two points match. It does not mean the wire is dead.
Before and after you trust a zero reading, check the meter on a circuit you know is live. That is standard trade practice.
A clamp meter reads the current in whatever passes through its jaws. Around one wire, it reads that wire's current. Around the whole cable, the current going out and the current coming back cancel, so it reads zero.
Never open a live circuit to put a meter in line to read amps. Use a clamp meter around one wire.
Testing a live circuit is a trained task, and so is working in a panel with the cover off. Wear rubber insulating gloves with leather protectors and safety glasses. Add arc-rated clothing and a face shield when the panel's arc flash label calls for them. This is NFPA 70E practice, not an NEC rule.
Use a meter and a clamp meter rated CAT III 600 V or higher for work in a panel. The CAT rating comes from the IEC 61010 product standard, not the NEC.
This is a subpanel. A feeder from the main panel brings two hots, a neutral, and a ground. The hots land on the main lugs.
In a subpanel, neutral and ground stay apart. The neutral bar sits on insulators so it does not touch the case. The ground bar is screwed to the case. See 250.24(B) and 408.40.
The model leaves out the small voltage the feeder loses. So the neutral and ground bars in the subpanel read the same, and the 1.2 volts from neutral to ground all comes from the branch circuit.
The cable is 12 AWG copper from a 20 amp breaker. It runs 50 feet to the receptacle with the meter in it, then 25 more feet to the heater's receptacle.
Copper wire has a little resistance. 12 AWG copper has 1.93 ohms per 1,000 feet, from Chapter 9 Table 8. So 50 feet of neutral has 0.097 ohms, and 12.1 amps through it loses 1.2 volts.
The heater is rated 1,500 watts at 120 volts, so it acts like 9.6 ohms. It gets about 116.5 volts here, so it draws 12.1 amps instead of 12.5.
In this model, the subpanel stays at 120 volts. A real reading runs a little above or below. The cord, the plug, and the receptacle contacts are left out of the math.
The meter draws almost no current, so it does not change what it reads.
The red lead plugs into the meter's V jack and the black lead into COM.
Readings are rounded. That is why 118.8 minus 1.2 comes to 117.6, not the 117.7 the meter shows.
The dots show how much current flows. More dots passing each second means more amps. The dots do not show how fast electrons move. Real electrons drift far slower than a snail, a fraction of a millimeter per second in house wiring.
Wall power is AC. It runs 60 cycles a second, so it reverses 120 times a second. That is too fast to see. The dots show the current going out on the hot and back on the neutral. Both meters are set to read AC.
The cable and the cord are drawn cut open to show the wires inside. Cable jacket colors show the wire size by maker custom, not by code. Yellow is 12 AWG.