408 



ELECTRICITY. 



of 4.9 per cent., to 19.47 in the scale. In con- 

 clusion, the authors state that they find no al- 

 loy of copper which conducts electricity better 

 than pure copper ; and they call the attention 

 of experimenters to the importance of stating 

 in future determinations whether the wires 

 they employ are hard drawn or annealed, and 

 at what temperature the observations are made, 

 A more full abstract of the paper appears in 

 the " Jour, of the Franklin Inst.," for May, 

 1862. 



Electric, Lights for Lighthouses. Mr. J. K. 

 Whilldin, C. E., communicates to the journal 

 just quoted (April, 1862), an article relative to 

 Prof. Way's electric light, with mercury, and 

 to the arrangements for projecting lights gener- 

 ally. Before entering upon the topics treated 

 of by him, it may be remarked that the intro- 

 duction of the electric light for lighthouse pur- 

 poses appears to have been directly owing to 

 the demonstrations by Prof. Faraday, in 

 connection with Prof. F. H. Holmes, of the 

 practicability of its use, and especially to the 

 experimental exhibition of its capabilities in 

 the South Foreland lighthouse, near Dover. 

 The electric light is obtained by heating to in- 

 candescence, by means of the passage of an 

 electric current between or through the bodies 

 so employed, carbon points, which are made to 

 terminate the positive and negative conducting 

 wires of a battery, or a slender filament of some 

 metal introduced between the ends of the wires. 

 In the former case incandescence of the points 

 can be produced only when these are brought 

 very close together, but usually not into posi- 

 tive contact ; in the latter, the light is the re- 

 sult of the intense heating of the fine wire 

 owing to the low conducting power it possesses 

 for the current. In either case, a current of 

 very high intensity is usually required; and 

 this may be obtained either from a galvanic 

 battery of many cells, say from 40 to 100, or 

 from the magneto-electric machine. The suffo- 

 cating nitrous fumes generated during action 

 of a Bunsen's battery, and indeed, the expense 

 of materials and the attention required in order 

 with any form of battery to maintain a regular 

 and intense current, constitute serious objec- 

 tions to their employment. Prof. Faraday 

 gave preference to the electro-magnetic ma- 

 chine, as being both a less troublesome and a 

 more economical source of electricity than any 

 galvanic battery. At the present time both 

 are, in different places, in use. The current of 

 the electro-magnetic machine is instantly pro- 

 duced upon giving a rotary movement to the 

 mechanism, regular while the movement is 

 kept up, and at once discontinued, without 

 waste, when it ceases. Its use thus involves 

 the addition of some motive power ; so that 

 where a steam engine is required for other 

 purposes, this mode of producing the current is 

 readily and inexpensively resorted to. "Where 

 such an engine is not present, Mr. Whilldin 

 suggests the economy and advantage of Erics- 

 son's hot-air engine. 



Mr. Whilldin calls attention to the great cost 

 of the Fresnel lenses now generally employed 

 for projecting the lights, of whatever kind; 

 those of the first order, as at Cape May, Hatte- 

 ras, &c., 6 ft. diameter by 9 ft. high, costing 

 from $5,000 to $11,000. By employment of the 

 electric light he judges that for these an appara- 

 tus not costing more than $400 to $500 can be 

 substituted. Faraday, indeed, states that all the 

 light from an electric lamp could be utilized 

 within a space not exceeding the size of an ordi- 

 nary hat, a result that, if practicable, would re- 

 duce the cost yet more. The difference and 

 saving arise mainly from the diminution allowed 

 in the size of the very costly lens arrangements, 

 as the electric light is produced almost at a 

 point ; while to augment the intensity and pene- 

 trating power of the common oil lights, the 

 number of the burners and the space occupied 

 by the flame must be very greatly increased. 

 With the electric light, increase of brilliancy in- 

 volves increase of battery power, and so of ex- 

 pense in this way, but with no enlargement or 

 inconvenience in respect to the lenses. 



The electric light hitherto most commonly 

 employed for lighthouses, has been that of the 

 carbon points. In this system the difficul- 

 ties practically encountered have been chiefly 

 those of obtaining the carbons sufficiently free 

 from impurities and variations of compactness 

 to preserve a uniform current and brightness, 

 and so to maintain the proximity of the points 

 by clockwork or other automatic mechanism 

 during their slow but continued waste, as to 

 prevent interruptions of the current from this 

 source, either through too great distance or 

 absolute continuity of the carbon electrodes. 

 Prof. Holmes' magneto-electric light, however, 

 with carbon points, appears from evidence in a 

 late parliamentary report to have been success- 

 fully used in the South Foreland lighthouse,Eng- 

 land, during a period of 6 consecutive months. 

 In France, also, magneto-electric lights are in 

 successful use ; and though there the system is 

 peculiar in the respect of continually reversing 

 the current, no important difference in the two 

 lights has been detected. 



By means of dark glass, Prof. Faraday com- 

 pared the electric light with that of the sun: 

 the latter was not at the time at its brightest, 

 but the intensities of the two lights were about 

 equal. He says that the eyes of the keepers of 

 the South Foreland light are not affected,though 

 protected by blue glasses of very pale color; 

 but that they judge better of the light by ob- 

 serving the place and intensity of the rays 

 within the lantern, than by looking at the light 

 itself. In some experiments on lights in France, 

 the intensities of an argand burner and of the 

 electric light were found with approximate cor- 

 rectness to be as 1 and 94; and that of the 

 " first-order flash" of the former being 80 to 90, 

 that of the "cast-glass flash" of the latter was 

 placed at 55,000, and of the " first-order flash" 

 at 220,000. The electric light is particularly 

 valuable for its power to penetrate a thick, 



