The Lamp Room

Why arcs in Jacob's Ladders tends to be dim compared to the superglaring carbon arcs and welding arcs?

The power supplied to the arc is different

Most JL are made to a minimal power level sufficient to produce a visible arc, hot enough to stay stable and rise up. Wear of the electrodes and emissions of metal fumes from the electrodes into the ambient air are to be minimized as well

Welding and lighting are applications where the actual power of the arc is of interest. In the case of lighting, there is also interest to make an arc stable enough to minimize flicker, which means fairly thick arc. In the case of welding, there is also interest in the arc's ability to deposit material from one electrode to another. So they are made to produce a high power arc

It is possible to build a high power JL and low power carbon arc lamp or welder, but their performance will be accordingly

In addition, here comes the technical question of how to start the arc. In JL the voltage must ionize the air on its own, just by strength of electrical field, at the start of a cycle. This requires voltage in excess of 3kV/mm of arc

In welding and lighting, the arc can be started from the condition when the electrodes are shorted and moved apart, which means that the voltage must not be too high, and the balance is more towards the current

Also, in the case of welding by a manual welder, there is interest to keep the voltage as low as possible to reduce the risk of electric shock to the operator

I didn't know about that.

If you are playing with Jacob's ladder and have access to some pure (sodium-free) lithium salt like LiCl, LiBr or LiI, try painting JL electrodes with it, mixed with a little bit of water. Just a small layer is enough. Result is just breathtaking, a beautiful sharp bright pink arc, million times better the the arc in a plain air.

Isn't lithium have its resonance line at the deep red and additional line at the red-orange line?

Sure main lithium lines are in deep red and orange, so it colors the arcs reddish not only in lamps, but also in the free air. Nitrogen still radiates some blue light too, and the resulting color is a bright pretty pink.