50 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVIII.) 
principles which are concerned in their production. Hence the value I think of the 
moving' wire as an investigator ; for it leads us into inquiries which touch upon the 
very nature of the magnetic force. There is no doubt that the needle gives true ex- 
perimental indications ; but it is not so sure that we always interpret them correctly. 
To assume that pointing is always the direct effect of attractive and repulsive forces 
acting in couples (as in the cases in question, or as in bismuth crystals), is to shut 
out ideas, in relation to magnetism, which are already applied in the theories of the 
nature of light and electricity ; and the shutting out of such ideas may be an obstruc- 
tion to the advancement of truth and a defence of wrong assumptions and error. 
3157. What is the idea of polarity in a field of equal force ? (whether it be occupied 
by air or by a mass of soft iron?) A magnetic needle, or an oblong piece of iron, 
would not show it in the air or elsewhere, except by disturbing the equal arrange- 
ment of the force and rendering it unequal ; for on that the pointing of the needle 
or the iron, or the motions of either towards the walls of the magnetic field, if limited 
(2828.), would depend. A crystal of bismuth in showing this polarity by position 
(2464. 2839.), does it without much altering the distribution of the force, and the 
alteration which does take place is in the contrary direction to that effected by iron 
(2807.), for it expands the lines of force. It seems readily possible that a magne- 
crystal might exist, which, when in its stable position, should neither cause the con- 
vergence nor divergence of the lines of force within it. It need only be neutral in 
relation to space or any surrounding medium in that direction, and diamagnetic in 
its relation in the transverse direction, and the conditions would be fulfilled. 
3158. But though an ordinary magnetic needle* cannot show polarity in a field of 
equal force, having no reference to it, and in fact ignoring such a condition of things, 
a moving wire makes it manifest instantly, and also shows the full amount of mag- 
netic power to which such polarity belongs ; and this it does without disturbing the 
distribution of the power, as far as we comprehend or understand distribution, when 
thinking of magnetic needles. At least such at present appears to me to be the 
case, from the consideration of the action of thin and thick wires (3141.) and wires 
of different substances (3153.). 
3159. As an experimentalist, I feel bound to let experiment guide me into any train 
of thought which it may justify ; being satisfied that ex- 
periment, like analysis, must lead to strict truth if rightly 
interpreted ; and believing also, that it is in its nature 
far more suggestive of new trains of thought and new 
conditions of natuial power. In order to extend its 
indications, and vary the form in which the principle of 
the moving wire may be applied, I had an apparatus con- 
structed, fig. 19, consisting of a wooden axis, one ex- 
tremity of which was terminated by a copper screw, 
* One could easily imagine hypothetically a needle that should do so. 
Fig. 19. 
