I have a bunch of old two-prong cube taps that are perfectly good, but they're not polarized (one opening wider than the other), and won't accept modern, polarized plugs. There's an easy way to polarize them, though, which I'll illustrate shortly, but a little background information is in order first.
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Operationally, an AC (alternating current) appliance/device is polarity insensitive -- the current is reversing direction sixty times per second, so the notion of a positive/negative polarity as you have with a car battery is meaningless; whichever way the AC supply is connected, the appliance/device will work normally. However, there is a safety consideration.
A two-conductor AC power cord has a neutral and a line (hot) conductor. The neutral wire is safe to touch -- it's ultimately connected to earth ground. The line wire is the dangerous one -- that's the energized conductor. A device such as an incandescent lamp can be wired with 'polarity', so that the likelihood of a user contacting a 'hot' surface is minimized[1]. For such a scheme to be effective, the device's plug must be polarized so that line and neutral are always and only present where they're supposed to be. That's the reason for the modern, 'polarized', two-prong plugs and receptacles.
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Ok. With that out of the way, let's return to the matter of polarizing an old, unpolarized cube tap.
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Pictured below is one of my cube taps, along with a polarized plug.
Note the wider one of the plug's two prongs -- that's the neutral. Note the equal-width openings on the cube tap -- the tap won't accept that modern plug.
The cube tap is made of a hard plastic that machines easily. Here's how to go about polarizing such a tap:
a) With a Sharpie marker, label the tap's prongs 'L' and 'N'; the one labelled 'N' you'll treat as the wide prong when you plug in the tap.
b) With an ohmmeter, find which of the tap's openings are common to your 'N' prong. Label those 'N', like so.
With a suitable diamond burr in a Dremel tool, widen the three 'N' openings so they'll accept the wide prong of a plug. Blow out the dust and you're done.
As long as you mind how you plug the tap in at first, you'll have a perfectly correct polarized cube tap.
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Note:
[1] A light bulb receptacle has a deeply recessed centre contact, and a large, easily accessible threaded sleeve portion.
Needless to say, any lighting fixture will be factory-wired such that the centre contact connects only to the line (hot) conductor of the power cord, and the threaded sleeve connects to the neutral.
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