Tap any part to name it. Tap a breaker, the wire nut, or a receptacle to work it. Drag a load to the other side.
Dots show one instant. Real current goes back and forth 60 times a second.
The box is drawn open to show the wires. On a job, test at the slots.
Loads are fixed resistances at their nameplate watts.
The old stereo can't take high voltage. It smokes above 150 V. Meters turn red outside 108 to 132 V. Both are teaching marks.
Meters here are 10 MΩ digital meters, so a loose wire can read a false voltage. Treat any reading as live until a low-impedance tester proves it dead (trade practice).
12 AWG copper in NM cable is good for 20 A: 334.80(A), Table 310.16. 20 A breakers: 240.4(D).
The glow on the white wire shows current over its rating. Real wire runs hot; it doesn't glow.
The code calls a shared neutral circuit a multiwire branch circuit: Article 100, 210.4(A). Both hots shut off together: 210.4(B). Single breakers need a handle tie: 240.15(B)(1).
Pulling a receptacle must not open the white wire: 300.15(B).
With both hots on one leg there is no voltage between them, so it isn't a multiwire circuit, and two circuits on one white wire break 200.6(A).
Grounds are spliced to each green screw and the box: 250.148.
Hands-on steps start with the breakers off and tested dead.
Kitchen counter receptacles also need GFCI, 210.8(A), and AFCI, 210.12(B). Both are left out here.