i873-] Magneto-Electric Illumination. 307 



III. MAGNETO-ELECTRIC ILLUMINATION. 



By William Crookes, F.R.S., &c. 



HE progress made in electric illumination during its 

 advance towards perfection has been several times 

 recorded in the pages of this journal. In our first 

 number, published nearly ten years ago, Dr. J. H. Gladstone 

 gave a history of the early difficulties attending the intro- 

 duction of the magneto-electric machine as a light-generator 

 for lighthouse illumination. Two years subsequently, the 

 present writer described Wilde's magneto-electric machine, 

 and, after a further lapse of years, during which time no 

 very important improvement in the industrial application of 

 magneto-electricity has been recorded, another step in 

 advance has been made which calls for detailed notice. 



The chief difficulties in the employment of magneto-electric 

 currents for industrial purposes have been their almost 

 instantaneous character and the rapid alternation in their 

 direction. The instrumental means necessary to seize hold 

 of these rapidly alternating waves, and convert them into 

 a more or less continuous stream of force flowing in one 

 direction, are necessarily of a delicate character, and are 

 easily put out of adjustment. This is easily understood 

 when it is remembered that in the machine first tried by 

 Mr. Holmes the rubbing surfaces were worn away in ten or 

 twenty minutes. The Berlioz machine required for its 

 maximum of intensity 350 or 400 revolutions per minute, 

 and the direction of the current is then reversed nearly 

 6000 times per minute ; here, however, the alternate 

 currents are not brought into one.* In the machine made 

 by Mr. Wilde for the Commissioners of Northern Light- 

 houses, the first armature is made to revolve about 2500 

 times a minute, generating 5000 waves of electricity. 

 These alternate currents are converted into an intermittent 

 current moving in one direction only by means of a com- 

 mutator. The second armature revolves 1800 times a 

 minute, generating 3600 alternately opposed waves of 

 electric force, which are picked up and sent in one direction 

 by a commutator, as in the former case.t 



It is evident that when a good friction contact is to be 

 kept between pieces of metal moving at these enormous 

 velocities, the wear and tear is very great. For a long time, 



* Quarterly Journal of Science, vol. i., p. 73. January, 1864. 

 f Ibid., vol. iii., p. 504. O&ober, 1866. 



