In the "Discovery Center" there was a fun little electricity demo that let you construct simple parallel and series circuits with some magnetic, conducting building blocks. There was a light bulb block, a buzzer block, and an LED block (that glowed green when the current flowed in one direction and glowed red when the current flow in the other). Neat exhibit. The power supply was mounted above the desk with the circuit blocks, and it let you choose to power your circuit with two, four, or six AA batteries. A large knob in the center of the panel selected the power source, and two analog meters displayed the voltage and current.
Here's the setting for 3V output (2 AA cells):
Here's the setting for 6V output (4 AA cells):
Here's the setting for 9V output (6 AA cells):
See the problem? The voltmeter and the ammeter are both wired in series. As connected, this circuit couldn't possibly work! Here's a clearer schematic:
WRONG. The ammeter measures the current through the batteries, but the voltmeter must measure the voltage ACROSS them. The voltmeter should be connected in parallel with the batteries, not in series. The panel should be wired like this schematic:
Better yet, to emphasize that voltage is measured across the power source, and that current is measured through it, the panel should be redesigned to look like this schematic:
Of course, the orange wires in the display case are just representational, and the batteries shown aren't really connected to anything (there is a wall-powered power supply behind the panel), but this error in the connection of the voltmeter should be embarrassing. This exhibit would be a good chance to discuss "across" variables and "though" variables to more advanced students, but the "artist" who designed the panel blew it.
UPDATE: See the responses in the comments below!