25 



two binding screws of this little motor, motion is produced and 

 continues until I disconnect the terminals. Here we see that the 

 energy of electric currents has been converted into the energy of 

 mechanical motion, and we have, according to our definition, a 

 .Dynamo. 



There is one exception that must be made to the definition just 

 given. All machines made for the production of statical electricity, 

 Hke those of Wiushurst and Hotz, must be excluded. 



A few simple experiments will enable us to understand better the 

 principle of the Dynamo. On the table we have the two kinds of 

 magnets — the permanent and the temporary. The former is made 

 of steel, and when once magnetised, retains its magnetism, while 

 the latter is made of soft iron, and is only a magnet when the 

 electric current is sent round it ; this is owing to the fact that 

 what is known as " coercive force " is greater in steel than in soft 

 iron. 



On this stand is poised a magnetic needle, perfectly free to move 

 either way in the horizontal plane. When left to itself it takes up 

 a position nearly north and south ; the end which points north is 

 marked by being coloured red. If the north end of another magnet 

 be brought near this coloured end there is repulsion, but if brought 

 near the other end there is attraction. Thus we learn that north 

 repels north, but attracts south, giving rise to the well-known rule 

 " Like poles repel, unlike attract." 



Now a magnet is able to exert its force to a considerable distance 

 all round it ; this is evident from the experiments I have just 

 performed, for the horizontal magnetic needle is affected long before 

 another magnet is brought close to it. This force exerted by the 

 magnet is very plainly seen by covering one with a sheet of paper 

 or glass, and then dusting over it some iron fiHngs. We have an 

 illustration of this on the table ; by means of the lantern we have 

 the same experiment on a larger scale. The iron filings lie in tufts 

 at the ends, and they appear to curve round from one pole to 

 another. These curved lines are called " Lines of Force," and the 

 whole area, occupied by these, is called the magnetic field. These 

 lines of force are always present round a magnet, and by the use 

 of iron filings their presence is made known to us. If the poles of 

 two magnets, or the two poles of one magnet, be brought near each 

 other, the disposition of the lines of force can be plainly seen by the 

 free use of the iron filings. Not only do these lines of force sur- 

 round a magnet but every electric current is surrounded as well by 

 a magnetic field, which, if time permitted, I could prove to you by 

 nsing iron filings. 



If a piece of soft iron be placed near a magnet, it also becomes 

 a magnet for the time, but of the opposite polarity. Thus by hold- 



