ON STANDARDS OF ELECTRICAL RESISTANCE. 133 
the ends of long magnets; and this conception of a magnet, as having two 
poles capable of exerting opposite forces joined by a bar exerting no force, is 
so much the most familiar that we shall not hesitate to employ it, especially 
as many of the properties of magnets may be correctly expressed in this way ; 
but it must be borne in mind, in speaking of poles, that they do not really 
exist as points or centres of force at the ends of the bar, except in the case of 
long, infinitely thin, uniformly magnetized rods. 
If we mark the poles of any two magnets which possess similar qualities, 
we find that the two marked poles repel each other, that two unmarked poles 
also repel each other; but that a marked and an unmarked pole attract each 
other. The pole which is repelled from the northern regions of the earth is 
called a positive pole; the other end the negative pole. The negutive pole is 
generally marked N by British instrument-makers, and is sometimes called 
the north pole of the magnet, whereas it is obviously similar to the earth’s 
south pole. 
The strength of a pole is necessarily defined as proportional to the force it 
is capable of exerting on any other pole. Hence the force f exerted between 
two poles of the strengths m and m, must be proportional to the product m m,. 
The force, f, is also found to be inversely proportional to the square of the 
distance, D, separating the poles, and to depend on no other quantity ; hence 
we have, unless an absurd and useless coefficient be introduced, 
mm, 
— D2 . . . . . . . . « . . (1) 
From which equation it follows that the unit pole will be that which at unit 
distance repels another similar pole with unit force; f will be an attraction 
or a repulsion according as the poles are of opposite or the same kinds, The 
L? MP 
dimensions of the unit magnetic pole are 
6. Magnetic Field.—It is clear that the presence of a magnet in some way 
modifies the surrounding space, since any other magnet brought into that 
space experiences a peculiar force. The neighbourhood of a magnet is, for 
convenience, called a magnetic field; and for the same reason the effect pro- 
duced by a magnet is often spoken of as due to the magnetic field, instead of 
to the magnet itself. This mode of expression is the more proper, inasmuch 
as the same or a similar condition of space may be produced by the passage 
of electrical currents in the neighbourhood, without the presence of a magnet. 
Since the peculiarity of the magnetic field consists in the presence of a certain 
force, we may numerically express the properties of the field by measuring 
the strength and direction of the force, or, as it may be worded, the intensity 
of the field and the direction of the lines of force. 
This direction at any point is the direction in which the force tends to move 
a free pole; and the intensity, H, of the field is necessarily defined as propor- 
tional to the force, f, with which it acts on a free pole ; but this force, f, is also 
proportional to the strength, m, of the pole introduced into the field, and it 
depends on no other quantities ; hence 
; STEEL, (ine tal oie ey Ue ot Cee 
and therefore the field of unit intensity will be that which acts with unit 
force on the unit pole. 
M? 
1aT 
The lines of force produced by a long thin bar-magnet near its poles will 
The dimensions of H are 
