Point at or tap any part to name it. Tap two of the circled points to move the meter probes.
This is single-phase: one coil, one wave. B only looks flipped because both legs are measured from the middle.
The meter shows the RMS value, 120 V. The wave's peaks reach about 170 V.
Each leg swings back and forth 60 times a second. The graph and dots are slowed down.
Blue: the leg is above the neutral. Purple: below. Deeper color, higher voltage.
Dots show current rocking back and forth. Bigger current, bigger swing.
Loads: lamp 100 W, toaster 1200 W, water heater element 4500 W at 240 V. Breaker sizes are typical.
Thermostats and the heater's second element are left out. A real element under water does not glow.
On "Heater on one leg", the fix is a 2-pole breaker with one pole on each leg. A tripped high-limit or open thermostat also reads 0 V across the element.
The heater's white wire is re-marked as a hot with tape at both ends, 200.8(C)(1).
120/240 V is a nominal system voltage, 120.5(A). Article 100 calls the coil's middle the neutral point.
The neutral is the wire that gets grounded, 250.26(2). It is bonded to the panel at the service, 250.24(C), and not again after, 250.24(B).
One ground rod is shown; one rod usually needs a second electrode, 250.53(A)(2).
Leg B is drawn red; on a real service drop both hots are black. Current through the earth is left out.
The main lugs stay live even with the main breaker off.
Covers and plates are off so you can see the wiring. On the job they go back on before the power does.