28 Electric Light 



or utterly destroying them. The action of a dynamo machine is 

 as follows: — When the armature is caused to rotate, the residual 

 magnetism in the iron induces a feeble current in the revolving 

 coils ; this current passes along the wire encircling the magnets, 

 and strengthens the magnetism, which in turn induces a stronger 

 current. Thus an action and reaction taket place, but with such 

 amazing rapidity that practically the machine is enabled to 

 generate its maximum strength of current instantaneously. 



I have referred to both the " arc" and " incandescent" forms 

 of electric light. The former is that produced by the separation 

 of two carbons after the current has been established ; it meets 

 with great resistance at the point of separation, and thereby 

 heats up the ends of the carbon to an enormous temperature, 

 thus producing a light of intense brilliancy. Both carbons 

 consume away, but not at the same rate. The one at which the 

 current enters from the machine, and called the " positive" 

 carbon, is consumed twice as fast as its neighbour or "negative" 

 carbon. The "positive" has a concave or hollow-shaped end, 

 whereas the negative is pointed. A portion of the positive is 

 carried to the negative by the action of the current. This is 

 only the case when direct currents are used, but with alternating 

 currents both carbons consume alike. 



With " incandescent" lighting, the lamp consists of a small 

 glass globe, from which all the oxygen has been exhausted. 

 Inside the globe there is a fine filament of a carbonised material, 

 made by different inventors from various products, but in the 

 final condition reduced to carbon. The current traverses this 

 filament, which being of considerable resistance, becomes heated 

 to whiteness, and thus gives off a beautifully clear and soft light 

 With reference to the power required to drive an electric 

 machine employed for generating currents of electricity for 

 " arc" or for " incandescent" lighting, the same power will 

 produce about ten times the aggregate light with an "arc" 

 compared to " incandescent," hence it is more economical where 

 large spaces have to be illuminated ; but for confined spaces, 

 especially where there is not much head room, the arc light is 

 far too brilliant. Under these conditions, the loss of power can 



