Old DC rectifier in Cinema

[AJshep]

I carry out some maintenance in a local cinema which is now 100 years old and every time I go into the projection room I look at this old decommissioned rectifier and realise how easy things are for use nowadays with all our tools and modern electronics.

I'm guessing this rectifier was built to control fluorescent lights around the screen, there are dials on the front Strike Burn and Run (I think) there is also the large mechanical leaver dimmers on the back wall which I guessed the projectionist would control.
I understand it was common practice to fuse both line and neutral, and have read tthat it was common for someone to reverse polarity every so often to stop tubes darkening at one end?

Anyone know much about these?

 

[DPG]

Lovely!

I reckon Lucien will be able to fill in the gaps here.

 

[James]

@Lucien Nunes
this is his sort of thing,
dont take pictures of the internals, to some that is just electrical ----!!

 

[AJshep]

@Lucien Nunes
this is his sort of thing,
dont take pictures of the internals, to some that is just electrical ----!!

Next time I go there I will take off the lid and update this thread.

 

[Avo Mk8]

I defer to Lucien, hoping he confirms the dc supply would be for the carbon arc lamp in the projector.

There wouldn't be fluorescent lamps back then (not until mid 30's I think )The rheostats/dimmers would surely be for dimming the filament house lights?

I wonder if the rectifier itself is still there somewhere!

 

[James]

I defer to Lucien, hoping he confirms the dc supply would be for the carbon arc lamp in the projector.

There wouldn't be fluorescent lamps back then (not until mid 30's I think )The rheostats/dimmers would surely be for dimming the filament house lights?

I was thinking along the same lines but didn't want to make a fool out of myself by guessing wrong about stuff that is older than me!

 

[AJshep]

I defer to Lucien, hoping he confirms the dc supply would be for the carbon arc lamp in the projector.

There wouldn't be fluorescent lamps back then (not until mid 30's I think )The rheostats/dimmers would surely be for dimming the filament house lights?

I wonder if the rectifier itself is still there somewhere!

This makes more sense! Thanks for the info.

 

[Lucien Nunes]

Before the advent of xenon lamps, the most intense electric light source was the carbon arc and these were universally used for cinema projectors, studio lights and searchlights. Carbon arcs work by vaporising carbon (and other elements) from a solid rod and passing current through the vapour to form a plasma. Although they can run on AC, the steadiest and brightest light requires DC at a roughly constant current, with typical projection arcs using between 40 and 120 amps depending on size. The voltage needs to be allowed to vary as this is dependent on the arc length which is under automatic adjustment, usually in the range of 30-60V.

Before the advent of silicon rectifiers, making DC at this range of currents and voltages from the mains was hard work. If the mains were DC, as many were before WW2, the usual solution was a motor-generator, with e.g. a 240V DC motor running from the mains, driving a 100V DC generator. The 100V was then ballasted by a tapped resistance to control the current and absorb the variations in arc voltage. The same method could be used for AC mains, but from the mid 1930s a more popular solution was the mercury-arc rectifier (see my avatar) as these are silent and maintenance-free. Metal rectifiers and oxide-cathode rectifiers were also sometimes used.

All cinemas had at least two projectors, sometimes more, plus Brenographs, followspots etc in cine-variety venues. Where there were many arcs to power, a typical setup would have two rectifiers feeding a common 110V DC bus with separate control resistances for each arc. Arc resistances were often placed outside the projection booth and remote controlled, due to the amount of heat dissipated. Alternatively, if the mains were AC and only two projectors needed supply, a more efficient method was to equip each with its own rectifier and do the ballasting with a tapped choke on the AC primary side, avoiding any resistive loss. In both the above setups, you would have an arc control of some kind next to the projector or mounted on the pedestal or lamphouse shelf. There would be current control (sometimes coarse and fine,) voltmeter and ammeter, and then on the DC main circuit a DP isolator to ensure the arc was completely isolated for re-carboning.

Your arc control panel is for the 'No.1 arc" i.e. the arc on No.1 projector, and looks like a Hewittic unit. That would confirm it to have been the remote for a mercury-arc rectifier. Very few survive now; many were broken for their mercury when it was valuable (it isn't any more) and later they were removed due to the increased concern about hazardous spillage if broken.

Operational carbon-arcs are scarce now too and carbons are hard to find. Only one or two manufacturers will still make illumination carbons (as opposed to brazing or cutting) and then only to order. I have a stock that I have hoarded so I still occasionally burn arcs for fun. Yeah, video, I know...

 

[AJshep]

Wow, that's awesome thanks for the detailed response. It appreciated!!

 

[ruston]

Operational carbon-arcs are scarce now too and carbons are hard to find. Only one or two manufacturers will still make illumination carbons (as opposed to brazing or cutting) and then only to order. I have a stock that I have hoarded so I still occasionally burn arcs for fun. Yeah, video, I know...

My first welder when I was a lad was carbon arc , not very successful. Later in life I used carbon rods for gouging.

 

[Lucien Nunes]

And you can still buy carbons for exactly that purpose, but they are no good for illumination. A cinema or studio carbon is quite complex, consisting of a shell and a core of different compositions, different for positive and negative, with the core of the positive carbon requiring carefully controlled chemistry. Light is produced by a combination of incandescence caused by resistive heating, combustion of the carbon and combustion of the additives that are selected to balance the colour spectrum for a particular application. In the heyday of the super-cinema, 2-3000 patrons might be sitting, watching, spellbound by the images formed with light from that tiny, white-hot crater in the end of the positive. It was big business and no expense was spared on R&D by the big carbon makers (e.g. Morganite and Ship in the UK) to get the best possible results.

It is hard to describe how intricately involved you get with the light when burning an arc. It's a living flame that repays constant supervision and adjustment (aka trimming) as well as a sound understanding of how the different parameters interact. And it's just one of the activities that was constantly going on in the projection box as the projectionist(s) went about their work, of which the patrons hopefully remained blissfully unaware.

 

[R-fur]

When I was an apprentice I worked with a guy who had been a projectionist in the fifties, He had lots of interesting stories about projection equipment, wish I had paid more attention now.