Hey guys, I recently grabbed a Welch Scientific vintage Mercury Vapor Spectral light source fixture from eBay for $30.  It has a GE (very heavy) ballast that has a metal stamped label stating (General Electric Autotransformer for H4 100 watt mercury vapor lamps, Primary 120vac 3 amps, Secondary 240 vac.  I tested it with a digital multimeter and it indeed has an open circuit voltage of 244vac with a line input of 120vac.  Here's the deal, I purchased it as a unique vintage way to hopefully display/observe the function of a Sylvania baby 100watt modern merc that's still available.  The fixture has a standard medium base socket made of ceramic and is on a display post that screws to the base.  It is heavy and mostly cast iron I'd guess from the 1950's.  it came with no bulb and by the look of the metal slit tube (removeable, covers bulb allowing slit light source, used in spectral exercises) the "H4" lamp must be a no longer produced tubular bulb.  So would I be alrite simply screwing in a modern day 100 watt coated or clear E17 shaped merc and enjoying it?  I trust you guys to have the knowledge :-)  Thanks ahead of time!

-Mike

What I have found, the H4 is indeed a 100W medium pressure lamp. As the electrical specs haven't changed, the ordinary 100W MV should match too. But the spectrum won't be as clear as with the medium pressure lamp (due to increase in the background continuum and in case of coated lamp of course the phosphor emission)

Thanks Medved!  Thanks a million.  I'm not ever going to use it for its original purpose of spectral use, simply to run up a clear lithonia 100w and a frosted sylvania 100w merc bulb for enjoyment lol. So it sounds like I'll be alrite!