ELECTRIC LIGHTING. 303 



tense induced currents, succeeding each other with gieat 

 rapidity, so as to be practically continuous, as with those pro- 

 duced by magneto-electric and dynamo- electric machines. 



All that I have said about the voltaic arc, the incandes- 

 cence resulting from resistance to the current's flow, and so 

 forth, in relation to electricity generated by galvanic bat- 

 teries, applies to electricity generated by induction coils, or 

 by magneto-electric and by dynamo-electric machines. Only 

 it is to be noticed that in some of these machines the cur- 

 rents alternate in direction with each revolution of the swiftly 

 turning coil, in others the currents are always in the same 

 direction, and in yet others the currents may be made to 

 alternate or not, as may be most convenient. 



We have now to consider how light suitable for purposes 

 of illumination may be obtained from the electric current. 

 Hitherto we have considered only light such as might be 

 used for special purposes, where a bright and very intense 

 light was required, where expense and complexity of con- 

 struction might not be open to special objections, and 

 where in general the absolute steadiness of the light was not 

 an essential point. But those who have seen the electric 

 light used even by the most experienced manipulators for the 

 illustration of lectures will know that the light as so ob- 

 tained, though of intense brilliancy, is altogether unsuited for 

 purposes of ordinary illumination. 



If we consider a few of the methods which have been 

 devised for overcoming the difficulties inherent in the pro- 

 blem of electric lighting, the reader will recognise at once the 

 nature of these difficulties, and the probability of their being 

 effectually overcome in the future, for though much has been 

 done, much yet remains to be done in mastering them. 



Let us consider first the Jablochkoff candle, the invention 

 of which brought about, in July 1877, the first great fall in 

 the value of gas property. 



The Jablochkoff candle consists of two carbons placed 

 side by side (instead of one above the other in a vertical 

 line). Thus placed, with a slight interval between them, the 



