That's a nice early model you have here Dave. If I remember correctly, in Sylvania's and GE's lamps the bimetal short-circuits de starting probe instead of disconnecting it. The very high impedance of the resistor (between 20 and 40 kOhm) draws only a minimum amount of current when it is switched in parallel to the burner during full regime.

Thanks for refreshing my memory! I knew many early MH lamps have the bimetallic switch, but I forgot their purpose. Now I remember reading about it in the 1975 GE HID guide.

no problem - in fact all probestart MH lamps, European or American, still have a bimetal switch. This is an essential device which is critical for preventing the problem of seal failure by sodium electrolysis.

Wow, Sylvania had the Metalarc family back then?! Metalarcs are very good lamps.

Metal Halide lamps have been around longer than you think. The 400w lamp was introduced in 1964. The 1000w and 1500w lamps came along in the very late 60s, and the 175w and 250w lamps in the mid 70s. The medium base MH lamps didn't come out until the 80s.

How come they don't make mogul base 35w HPS, 50w MH, 50w MV, 80w MV, etc? The base doesn't affect the lamp in any way does it?

There's one of those early metal halide lamps at school with all the lamps we use for our exercises. It's a Sylvania Canada with exactly the same design as yours, it has pretty much no hours on it.

There are also lots of very interesting lamps. In the lamp bin I found four Philips 400W mercury lamps with an unusual "Kleen Beam" name on them! Among the interesting finds, there's also a Sylvania Canada 175W clear mercury lamp, and a coated 70W HPS lamp that doesn't even ignite anymore.

The first televised color broadcasts of night sporting events (circa 1967) would have been difficult without 1000 & above watt metal-halide lamps!