720 THE POPULAR SCIENCE MONTHLY. 



tween light and the other form of force has not yet been established, 

 and it may not be going too far to conjecture that thermodynamics 

 may possibly in the future have to appeal to the action of light upon a 

 photographic plate. In the mean time we look forward to the promised 

 continuation of Dr. Yogel's researches with no little hope. — Nature. 



THE ELECTKIC LIGHT FOE STEAMSHIPS. 



By JOHN TEOWBPJDGE, 



ASSI8TA>'T PEOFESSOE OF PHYSICS AT HAEVARD ITNIVEESITY. 



THE employment of the electric light for general purposes of illu- 

 mination has not, hitherto, been successful. The difficulty of 

 maintaining it constant, and the expense attending its use, have pre- 

 vented its employment. In the old method of producing the light 

 by a great number of cells, the chief difficulties arose in keeping the 

 strength of the current constant, and in regulating the distance of 

 the carbon-points between which the light was produced. Certain 

 forms of the Daniell cell, notably that constructed by Sir William 

 Thomson (" Jenkins's Electricity," p. 223), give a sensibly constant 

 current for an indefinite period, if watched with great care. The so- 

 lutions of the cells, however, need to be carefully removed from time 

 to time. The distance of the carbon-points also can be regulated by 

 various contrivances, which do the work required of them in an ad- 

 mirable manner. Still, chemical action cannot be looked to as an eco- 

 nomical and constant source of the electric light. 



The remarkable improvements in magneto-electric engines have 

 led to another source of the electric light, and seem to afford a better 

 solution of the problem of its economical use. The principle which 

 underlies all magneto-electric engines can be briefly stated thus : The 

 movement of an electro-magnet in the neighborhood of a stationary 

 magnet, which may also be an electro-magnet, is sufficient to induce 

 a current in the coil of the first electro-magnet, and this current can 

 be exalted in strength almost indefinitely by its proper direction, and 

 by the rapidity of the mechanical movement. The most noted engines 

 are those of Siemen and Hulske, Wild, Ladd, and the Gramme ma- 

 chine. Some idea of the power of these engines can be gained from 

 the following statement in regard to a Wild machine, of a size in- 

 tended to be used for the production of the electric light for light- 

 houses : " When worked with a power of three horses, it will consume 

 carbon-sticks three-eighths of an inch square, and evolve a light of sur- 

 passing brilliancy. With a machine that consumes carbons half an 

 inch square, a light of such intensity is got, that, when put on a lofty 

 building, it casts shadows from the flames of the street-lamps a quar- 



