K. DOMESTIC AND HOUSEHOLD ECONOMY. 507 



electric machine. This machine produced the electricity- 

 necessary for the evolution of light by means of the rapid 

 revolution of permanent magnets ; and the great hinderance 

 to the perfect success of the apparatus consists in the diffi- 

 culty of attaining the extreme rapidity of revolution which 

 was required. 



The Berlioz machine revolved at the rate of three hundred 

 and fifty or four hundred revolutions per minute; in the 

 Wilde machine, the armature has sometimes made about 

 twenty-five hundred revolutions per minute. 



It is evident that at these high velocities' the wear and 

 tear of the material must be very great. It has, however, 

 been discovered that the successive and almost instantaneous 

 flashes of electricity produced so rapidly by this machine, as 

 to seem like a continuous light, can be somewhat lengthened, 

 so that extremely rapid revolutions are not necessary. The 

 difficulties sought to be removed by Mr. Wilde are now over- 

 come in a diffisrent manner by Mr. Gramme, and the Gramme 

 machine promises to more nearly attain the efficiency re- 

 quired of such apparatus than the Wilde machine. In the 

 former the magnets remain stationary, while the conducting 

 wire through which the electric current flows, or at least a 

 portion of that wire, is put in motion. These machines, 

 therefore, give an absolutely continuous current of electricity, 

 rather than a current intermitted a thousand times or more 

 per minute. The constancy of the strength of the current is 

 convincingly shown by the fact that in some recent experi- 

 ments carried on for eight hours with one of the first ma- 

 chines constructed, the deviation of the needle of the galva- 

 nometer was absolutely invariable. The power of the current 

 attained with a machine of a given siz^ may be estimated 

 from the following statements : 



A machine containing two electro -magnets worked by 

 hand decomposed Avater and fused a length of one inch of 

 iron wire y^ths of an inch in diameter. A large machine, 

 driven by a two and a half horse -power engine, produced 

 at a slow rate of revolution a light equal to eight thousand 

 candles. A machine driven by a four horse -power engine, 

 and itself four feet high, two fefet long, and two feet wide, 

 gave a light equal to nine thousand carcel lamps. In the 

 galvano-plastic works, these and similar machines have been 



