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, 
like those of Winshurst 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 roundit; 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 filings. 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 
using iron filings. 
If a piece of soft iron be placed near a magnet, it also becomes 
@ magnet for the time, but of the opposite polarity. Thus by hold- 
