A three-filament Nernst lamp gives as much as 1000 candles, but it is extremely hot, and the light rather diffuse. The filaments are also very fragile, so on the whole the lamp was never very much in favour; but on the other hand it consumed very little current, and could be worked from any ordinary house lighting main, points which led to its adoption in certain cases.
The Electric Arc.—We now come to the light for optical lantern work, the brightest, the most concentrated, the cheapest, the easiest to work, in fact, the illuminant which combines all the virtues and but few drawbacks, but of course requires one indispensable condition, viz. electric current laid on. This current may be of any voltage from 70-250, or even higher; it may be continuous or alternating, though the former is to be preferred; and it requires a cable for at least 5 ampères, and for a large hall 10 or 12 ampères.
The simplest form of arc lamp for lantern purposes is the
hand-fed type as illustrated in Fig. 26. The essential feature is the pair of carbon rods, the remainder of the apparatus consisting of mechanical adjustments to 'feed' these as they burn away, and to accurately maintain them in their proper positions and in the optical centre of the lantern. Just because the electric arc provides so small and concentrated a light, it is of extreme importance that the centring should be exact; and hence mechanical movements are usually provided for this which are unnecessary with other illuminants.
The whole question of optical adjustments has, however, been left over for a future chapter, as it more or less applies to whatever illuminant is used.
The illustration shows a lamp arranged for continuous current, the upper carbon, which must be connected to the positive wire, being larger than the lower (the negative), and very slightly behind it. The light from a continuous current arc lamp comes chiefly from this upper or positive carbon,
which 'craters' as it is used, and this arrangement has the effect of radiating the light in the direction required (Fig. 27).
The positive carbon is usually of the 'cored' type, that is provided with a core of softer carbon, as this assists the 'cratering' action, while the negative is generally used 'solid,' that is homogeneous right through.
The arc has to be 'struck' in the first place by touching the carbons together for a moment by the mechanical means provided, and then separating them to the working distance, which is approximately ⅛ inch. They must then be maintained at that distance by 'feeding' as they slowly burn away, and this 'feeding' in arc lamps for lantern work is usually done by hand, as in the lamp illustrated in Fig. 26, but may be done by an automatic arrangement, as will be described later.