A series circuit has one path. The same current flows through every heater at the same moment. The heaters do not take turns. Both clamp meters read the same.
To get the total resistance, add up the ohms of each heater. The current is 240 volts ÷ the total ohms. Each heater's voltage is the current × its ohms. Those voltages add up to 240 volts.
Each heater's ohms come from its nameplate, which is rated for 240 volts. Ohms = volts × volts ÷ watts. A 1,000 watt heater is 57.6 Ω, and a 500 watt heater is 115.2 Ω. Heater wire has nearly the same resistance hot or cold.
In a real house, each baseboard heater gets the full 240 volts on its own. They are wired in a chain here only to show a series circuit. In a chain, each one makes far less heat.
The glow shows each heater's heat compared with its full rating. Real heater wire this warm does not glow.
When a heater burns open, nothing heats, but the wires and screws are still live. The full 240 volts is across the break. Test before you touch. That is standard trade practice.
Only a trained person should read voltage on a live circuit, using a meter rated for the job.
The dots show current. More dots passing each second means more amps. The dots do not show how fast electrons move. Real electrons drift only a fraction of a millimeter per second.
Wall power is AC. The current goes back and forth 60 times a second. The dots move back and forth in place, slowed way down so you can see it. The meters show the usual effective (RMS) value.
The wires are drawn as perfect. Real branch circuit wires have a small fraction of an ohm, so they use up a little voltage.
Both hots leave the panel in one two-wire cable, which has a black wire and a white wire. The white wire is used as a hot, so it is marked with red tape at both ends, 200.8(C)(1).
You will meet series circuits on the job. A wall switch is in series with its light, so opening the switch stops the current. Old string lights were wired in series, so one burned-out bulb turned off the whole string.
The panel, breaker, and ground wire are left out. A real heater's metal case is grounded.
The wall and heaters are drawn cut open and not to scale.