The current is really carried across the arc by convection, or in other words conducted by a bridge of white hot carbon particles, which continually stream across from the positive carbon to the negative, and this bridge, while conducting the current, interposes a very considerable resistance (otherwise it would not of course become hot).
A certain potential or tension is therefore necessary if a given current is to be maintained, and this potential has to be greater the longer the arc and also (though not in direct proportion) the smaller the carbons.
When, however, everything is in the best proportion, i.e. length of arc, size of carbons, and current passing, the potential at the arc lamp terminals required is approximately 45 volts, and this may be taken as a fixed figure for any current.
The length of arc to give the best results may also be taken
as approximately fixed at ⅛ inch, and the variable factor for different currents as required is provided by altering the sizes of carbons employed.
The error must not be made, however, of assuming that an E.M.F. of 45 volts is sufficient to work an arc lamp, as the minimum in practice is at least 65 volts, and 100 or even 200 volts are advantageous.
I have come across more than one private generating installation where the innocent owner has put in a dynamo for 45 or 50 volts, depending upon some carelessly written statement that this is sufficient.
Why a higher E.M.F. is required can be simply explained.
Take for instance an average hand-fed arc lamp as used for lantern work and consuming, say, 10 ampères.