The consumption varies a little, but the average is from 3 to 4 inches per hour.
Concentration of Light.—Two kinds of concentrating apparatus are supplied in combination with the automatic lamp, both of which are capable of giving a powerful parallel beam, which will reach to an enormous distance, and are well adapted for naval purposes. The one kind consists of a parabolic reflector of stout metal, its concave surface being silvered and burnished. The apparatus is mounted with a ball-and-socket joint upon a wooden stand, as shown in [Fig. 171].
The other kind is the Fresnel catadioptric lens or holophote, [Fig. 172], which may be substituted for the reflector, and gives a more powerful beam than one given by reflection. The lens is surrounded by a metal case or lantern, in which is placed the electric lamp upon a slide for focussing. Behind the carbon points a hemispherical reflector is placed, to catch all the back rays, and reflect them back through the lamp focus. The entire lantern is capable of revolving on horizontal rollers, and swings upon pivots. Two handles are placed at the back to manipulate it.
As the electric arc is much too bright to be looked into with the naked eye, both concentrating apparatus are supplied with a lens, called a focus or flame observer, by means of which an image of the burning carbons is thrown upon small screens at the back, so that the lamp can be easily adjusted without fatigue to the eye. The focus observer is shown on the lamp in holophote, [Fig. 172].
Precautions.—Before starting the apparatus, the electric lamp terminals and those of the dynamo machine must be connected up by means of the leading wires provided with each set of apparatus. The terminals are marked C and Z respectively, and they should be connected, C of machine to C of the lamp, and Z of the machine to Z of the lamp, in order that the electric current may be sent in the proper direction through the carbons of the lamp. Should it, however, be found that the top carbon (which should consume twice as fast as that of the bottom one) does not consume so fast as the bottom one, it may be assumed that the dynamo machine has reversed its poles, and the leading wires will consequently require changing across. This reversal of poles, though possible, is of very rare occurrence.
