I think they are electric. The back of the gauge only has 2 wires, and no vacuum nipple. I think it's specific only to S15's too. You can use the holder in S14's though, but you have to cut the top bit of the plastic to fit.
Now if you've poured coolant into a radiator, you have no doubt that it is much thicker than water. That is, it has a much higher viscosity. This means that it...