Controls power to the motherboard and it must be connected for the PC to power up?
The power switch.
AT power supplies are connected to a motherboard with two power connectors, that must be aligned correctly. ATX power supplies use a single 20 or 24-pin connector that cannot be inserted incorrectly. AT power supplies are turned on and off with a physical switch. ATX power supplies are turned on and off by receiving a signal from the motherboard.
What technology would When a computer is being built which three components must have the same form factor?
The case, motherboard, and power supply must all be the same form factor. Obviously the motherboard and the case would have to be the same or the motherboard would never fit in the case. Then the power supply must not only fit in the case, but have the same connectors as the motherboard. Otherwise it won't work.
You can measure the current and power of a 'power supply', using an ammeter and a wattmeter. With the power supply connected to its load, the ammeter must be connected in series with the power supply's input. The wattmeter's current coil must also be connected in series with the power supply's input, and its voltage coil must be connected in parallel with the supply, taking the instrument's polarity markings into account.