238 THE ADDRESSES, LECTURES, ETC., OF 



Preece says that " if a mere crack or pinhole were to occur in an. 

 insulating coating the currents would escape to earth, generating 

 heat and producing fire," emphasizing this statement by asserting 

 that it is not a chimerical one, but the result of practical informa- 

 tion of his own. Now, the dynamo-electric current is powerful in 

 the sense of doing a large amount of work, but its intensity is so 

 moderate that it is only by rubbing the two unprotected conduc- 

 tors against one another that vivid sparks are produced. The 

 interposition of a shaving of wood or other inflammable substance 

 would in itself supply a sufficient insulating separator to prevent 

 the spark, and I challenge Mr. Preece, notwithstanding his well- 

 known skill as an experimenter, to the production of an igneous 

 discharge through a mere pinhole in the insulating covering of 

 one or other of the conductors. In practically arranging electric 

 lights for public buildings the insulated conductors are entirely 

 separate, and the likelihood of danger to buildings is absolutely 

 removed by the precautions taken. Mr. Preece's reference to the 

 unfortunate musician and Russian sailor who appear to have been 

 struck dead by electric discharges while assisting to put up electric 

 lights, is somewhat sensational. They must have been predisposed 

 to succumb to an electric shock, and must have grasped firmly the 

 two unprotected leading wires with moist hands ; for I and many 

 other persons have frequently touched both poles of a powerful 

 dynamo machine without feeling any the worse for it. At the 

 same time there are dangers connected with the establishment of 

 this, as of almost any apparatus or machinery, which do not in 

 any way apply to the user not set upon courting danger. Mr. 

 Preece also charges the electric light with being cold in appear- 

 ance, but in reality " the greatest source of heat which science 

 possesses." Now, I quite agree with him in this latter proposition, 

 and have, indeed, constructed an electric furnace for fusing highly 

 refractory materials ; but I wish to guard against the inference 

 that because the electric arc is hot, electric light must necessarily 

 heat rooms in which it is employed to anything like the same 

 extent as gas or, indeed, any other illuminant. The following 

 simple calculation, based upon actual experiment, will show what 

 is the relative heating effect of the two sources of light within a 

 room. To produce 4000 candle-power by electricity requires a 

 current of 34 Webers, which again represents 130 heat units per 



