79 o THE POPULAR SCIENCE MONTHLY. 



Some of it must always be wasted, and the true economist will try 

 and waste the least possible quantity. If I place a piece of platinum 

 wire between the two wires connected with the hand dynamo-machine, 

 you see that the muscular energy of my assistant, by turning the han- 

 dle, generates currents of electricity, which give a red heat to the 

 platinum wire. The energy of the body is thrown into the machine, 

 the machine converts it into electricity, the electricity passes through 

 the wire, which, by having work done upon it, is rendered incandes- 

 cent, and, in consequence, becomes luminous. I have here a lamp 

 containing a piece of platinum wire, and, if I connect it to the wires 

 of the dynamo-machine [this was done], the platinum glows and gives 

 us light. It is a machine precisely similar in principle to the one now 

 before you, fixed under the arches on the Thames embankment, and 

 worked by a steam-engine (lent for the purpose by Messrs. Robey) 

 that is supplying currents of electricity to the lamps now lighting this 

 room. There is an occasional throb in the light ; this is produced by 

 the unsteadiness of the engine, which was not specially prepared for 

 the purpose, but was the best available. It is, in fact, an agricultural 

 machine. There are two kinds of electric machines of this class. One 

 is called the magneto-machine, like the one before you, because the 

 magnetic field is produced by the presence of a powerful permanent 

 magnet, which, I think, is visible to most of you, and which consists 

 of several pieces of steel that have been magnetized. The other kind 

 of machine is called the dynamo-machine, in which the magnetic field 

 is produced by an electro-magnet, which is itself excited by the cur- 

 rents it generates, so that there is a kind of accumulative action ; one 

 current piles up the agony on the other current, and all of them 

 together, acting on the electro-magnet, increase the total effects, until 

 the iron is saturated with magnetism. So much as regards the pro- 

 duction of currents for electric-lighting purposes. The motion of the 

 conductor through the magnetic field may be caused by the energy 

 of coal, which is consumed to generate heat and steam for work- 

 ing a steam-engine ; or, as at Godalming, by the energy of water on a 

 water-wheel ; and it is very probable that, where water is available, it 

 will be the most economical source of energy for electric-lighting pur- 

 poses. Sir William Armstrong, at his seat at Craigside, near New- 

 castle-on-Tyne, has illuminated his house for some time by currents of 

 electricity, produced by a water-fall in his grounds, so that, he says, his 

 library and his drawing-room are lit by the river flowing through his 

 grounds. As regards the light itself, there are two kinds of lamps. 

 I have already explained and illustrated to you the fact that electric- 

 ity in its passage through air produces sparks. I have here what is 

 called an arc-lamp ; in it two rods of carbon are held by two brass 

 clips (not in metallic connection with each other), and the ends of the 

 carbon are, when in action, a short distance apart. On joining up the 

 wires to the brass clips the current flows, a bright light is instantly set 



