ELECTRIC LIGHTING. 301 



has been used for this purpose. If the conductive power of 

 copper be represented by 100, that of platinum will be 

 represented by 18 only. Thus the resistance experienced by 

 a current in passing through platinum is relatively so great, 

 that if the current is strong the platinum becomes intensely 

 heated, and shines with a brilliant light. A difficulty arises 

 in using this light practically, from the circumstance that 

 when the strength of the current reaches a certain point, the 

 platinum melts, and, the circuit being thus broken, the light 

 immediately goes out. 



The use of galvanic batteries to generate an electric 

 current strong enough for the production of a brilliant light, 

 is open to several objections, especially on the score of ex- 

 pense. It may, indeed, be safely said that if no other way 

 of obtaining currents of sufficient intensity had ever been 

 devised, the electric light would scarcely have been thought 

 of for purposes of general illumination, however useful in 

 special cases. (In the electric lighting of the New Opera 

 House at P. ris, batteries are used.) The discovery by Orsted 

 that an electric current can make iron magnetic, and the 

 series of discoveries by Faraday, in which the relation be- 

 tween magnetism and electricity was explained, made electric 

 lighting practically possible. One of these shows that if a 

 properly insulated wire coil is rapidly rotated in front of a 

 fixed permanent magnet (or of a set of such magnets), cur- 

 rents will be induced in the coil, which may be made to 

 produce either alternating currents or currents in one direc- 

 tion only, in wire conductors. An instrument for generating 

 electric currents in this way, by rapidly rotating a coil in 

 front of a series of powerful permanent magnets fixed sym- 

 metrically around it, is called a magneto-electric machine. 

 Another method, now generally preferred, depends on the 

 rotation of a coil in front of an electro-magnet ; that is, of a 

 bar of soft iron (bent in horseshoe form), which can be 

 rendered magnetic by the passage of an electric current 

 through a coil surrounding it. The rapid rotation of the 

 coil in front of the soft iron generates a weak current, because 



