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But whereas the general course of scientific method then 
consisted in the application of the ideas of mathematics and 
astronomy to each new investigation in turn, Faraday 
seems to have had no opportunity of acquiring a technical 
knowledge of mathematics, and his knowledge of astro- 
nomy was mainly derived from books. 
Hence, though he had a profound respect for the great 
discovery of Newton, he regarded the attraction of gravi- 
tation as a sort of sacred mystery, which, as he was not 
an astronomer, he had no right to gainsay or to doubt, his 
duty being to believe it in the exact form in which it was 
delivered to him. Such a dead faith was not likely to 
lead him to explain new phenomena by means of direct 
attractions. 
Besides this, the treatises of Poisson and Ampére are 
of so technical a form, that to derive any assistance from 
them the student must have been thoroughly trained in 
mathematics, and it is very doubtful if such a training can 
be begun with advantage in mature years. ; 
Thus Faraday, with his penetrating intellect, his devo- 
tion to science, and his opportunities for experiment, was 
debarred from following the course of thought which had 
led to the achievements of the French philosophers, and 
was obliged to explain the phenomena to himself by means 
of a symbolism which he could understand, instead of 
adopting what had hitherto been the only tongue of the 
learned. : 
This new symbolism consisted of those lines of force 
extending themselves in every direction from electrified 
and magnetic bodies, which Faraday in his mind’s eye 
saw as distinctly as the solid bodies from which they 
emanated, ; 
The idea of lines of force and their exhibition by means 
of iron filings was nothing new. They had been observed 
repeatedly and investigated mathematically as an inte- 
resting curiosity of science. But let us hear Faraday 
himself, as he introduces to his reader the method which 
in his hands became so powerful.* 
“Tt would be a voluntary and unnecessary abandon- 
ment of most valuable aid if an experimentalist, who 
chooses to consider magnetic power as represented by 
lines of magnetic force, were to deny himself the use of 
iron filings. By their employment he may make many 
conditions of the power, even in complicated cases, visible 
to the eye at once, may trace the varying direction of the 
lines of force and determine the relative polarity, may 
observe in which direction the power is increasing or 
diminishing, and in complex systems may determine the 
neutral points, or places where there is neither polarity 
nor power, even when they occur in the midst of powertul 
magnets. By their use probable results may be seen at 
once and many a valuable suggestion gained for future 
leading experiments.” 
Experiment on Lines of Force 
In this experiment each filing becomes a little magnet. 
The poles of opposite names belonging to different filings 
attract each other and stick together, and more filings 
attach themselves to the exposed poles, that is, to the ends 
of the row of filings. In this way the filings, instead of 
forming a confused system of dots over the paper, draw 
together, filing to filing, till long fibres of filings are 
formed, which indicate by their direction the lines of force 
in every part of the field. ‘ : 
The mathematicians saw in this experiment nothing 
but a method of exhibiting at one view the direction in 
different places of the resultant of two forces, one directed 
to each pole of the magnet ; a somewhat complicated re- 
sult of the simple law of force. 
But Faraday, by a series of steps as remarkable for 
their geometrical definiteness as for their speculative in- 
genuity, imparted to his conception of these lines of force 
a clearness and precision far in advance of that with 
* Exp. Res, 3284. 
which the mathematicians could then invest their own 
formule. 
-In the first place Faraday’s lines of force are not to be 
considered merely as individuals, but as forming a system, 
drawn in space in a definite manner, so that the number 
of the lines which pass through an area, say of one square 
inch, indicates the intensity of the*force acting through 
that area. Thus the lines of force become definite in 
number. The strength of a magnetic pole is measured 
by the number of lines which proceed from it ; the electro- 
tonic state of a circuit is measured by the number of lines 
which pass through it. 
In the second place each individual line has a con- 
tinuous existence in space and time. When a piece of 
steel becomes a magnet, or when an electric current be- 
gins to flow, the lines of force do not start into existence 
each in its own place, but as the strength increases new 
lines are developed within the magnet or current, and 
gradually grow outwards, so that the whole system ex- 
pands from within, like Newton’s rings in our former ex- 
periment. Thus every line of force preserves its identity 
during the whole course of its existence, though its shape 
and size may be altered to any extent. 
I have no time to describe the methods by which every 
question relating to the forces acting on magnets or on 
currents, or to the induction of currents in conducting 
circuits, may be solved by the consideration of Faraday’s 
lines of force. In this place they can never be forgotten. 
By means of this new symbolism, Faraday laid down with 
mathematical precision the whole theory of electro-mag- 
netism, in language free from mathematical technicalities, 
and applicable to the most complicated as well as the 
simplest cases. But Faraday did not stop here. He went 
on from the conception of geometrical lines of force to 
that of physical lines of force. He observed that the 
motion which the magnetic or electric force tends to pro- 
duce is invariably such as to shorten the lines of force 
and to allow them to spread out laterally from each 
other. He thus perceived in the medium a state of stress, 
consisting of a tension like that of a rope, in the direc- 
tion of the lines of force, combined with a pressure in 
directions at right angles to them. 
This is quite a new conception of action at a distance, 
reducing it to a phenomenon of the same kind as that 
action at a distance which is exerted by means of the 
tension of ropes and the pressure of rods. When the 
muscles of our bodies are excited by that stimulus 
which we are able in some unknown way to apply 
to them, the fibres tend to shorten themselves and 
at the same time to expand laterally. A state of 
stress is produced in the muscle, and the limb moves. 
This explanation of muscular action is by no means 
complete. It gives no account ofthe cause of the excite- 
ment ofthe state of stress, nor does it even investigate 
those forces of cohesion which enable the muscles to sup- 
port this stress. Nevertheless, the simple fact, that it 
substitutes a kind of action which extends continuously 
along a material substance for one of which we know only 
a cause and an effect at a distance from each other, in- 
duces us to accept it as a real addition to our knowledge 
of animal mechanics, 
For similar reasons we regard Faraday’s conception of 
a state of stress in the electro-magnetic field as a possible 
method of explaining action at a distance by means of — 
the continuous transmission of force, even though we do 
not know how the state of stress is produced. 
But one of Faraday’s most pregnant discoveries, that ot 
the magnetic rotation of polarised light, enables us to 
proceed a step further. It has been pointed out by Sir 
W. Thomson that the phenomenon, when analysed into 
its simplest elements,can be expressed thus: that of two 
circularly polarised rays of light, precisely similar in con- 
figuration, but rotating in opposite directions, that ray is 
propagated with the greater velocity which rotates in the 
(Mar. 6, 1873 
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