What happens to diesel glow-plugs?

Here, the weather is cold and my diesel car has been getting hard to start. Traced the problem to the glow-plugs which I replaced. That made a huge difference with the engine starting on the first turn.

I thought I would run a few tests on the old glow-plugs. All four were not open circuit, so I connected them to a 12v battery. All four heated up and smoked.

One thing I did notice was that they all were low in current, 2-5 amps rather than about 15-20 amps I would have expected.

So what would cause this? Is this typical?