Faraday to George Biddell Airy   21 August 1854

Surbiton | 21 Aug 1854

My dear Sir

Your letter¹ sets the thoughts loose upon many points, none of which may be of the least consequence but you must take them for what they are worth;- talking would have dealt with them better than writing. In the first place unless the effects, & therefore the causes, are supposed to be very local I should imagine they would be as evident upon the surface as in the interior or nearly so. As respects general electric currents in the earth, they ought perhaps to be more evident when they are all below the thing affected, than when the latter is in the midst of them, for in the first case they would all act in one direction & in the second the parts on opposite sides would act in opposite directions.

If the pendulum has nothing magnetic about its moving parts then I do not see how earth-currents of electricity can affect it. If it were magnetic the currents might tend to turn it round an axis between the polar parts; but as such a force would act in the manner of a couple, I suppose it would not interfere with the time of vibration[.]

You speak of Magnetic currents, by which I conclude is intended electric currents. If the idea is to include the ordinary force of the earth, then a magnetic pendulum would be in different relation to it according as it swung in the plane of the magnetic meridian or at right angles to it, inasmuch as the pull would, in the first case, be all on one side of the swing; and in the other different in amount & direction & equally disposed on both sides:- but all that will occur to you at once; & that the effect will be the same as on the surface[.]

If the rocks surrounding the wire have any fixed local magnetic force, making the resultant of the earths magnetism in a given place, different to what it would be at the surface;- then I must leave you to judge what the effect of such a difference should be.

Let us now suppose the pendulum free from magnetism. If composed of non metallic materials then I have no more to say:- but if its bob be a mass of metal then the following considerations arise. The bob moves to & fro about a fixed point; and hence its upper and lower parts move through different spaces in the same time. In these latitudes, where the dip is very great, the consequence is, that moving across the earths lines of magnetic force, electric currents tend to be found; and because of the difference between the amount of lines intersected by the upper and lower parts of the bob, are really found, as I have shewn in my old researches², & often obtained since:- so that as the bob swings it will become, virtually, a very feeble electromagnet with a horizontal magnetic axis; i.e if the bob swings east & west in one direction it will have whilst moving, magnetic force in a north & south direction: as the bob returns in the contrary direction it will have a north & south magnetic force equal to the first amount but in the contrary direction. If swung in any other azimuth like results will occur, the vertical magnetic axis being at right angles to the plane of motion, provided the ball be solid & uniform[.]

I suppose this alternate magnetic condition (which must be very small in amount) will not interfere as such because of the reasons before given i.e because the effect will be that of a couple.

But the assumption of the state is accompanied by a resistance to the motion which generates it. It might possibly therefore have the effect of obstructing the motion:- whether that would only diminish the arc of vibration or whether it would affect the time also you can tell better than I can. The effect would be the same upon the surface of the earth and I directed Sabines attention to it a long while ago³.

And now that I speak of the surface I may point out another consequence of pendulum motion. If a pendulum were swung at the places where the dip was 90˚ whether in one azimuth or another would, magnetoelectrically, make no difference: but if swung where the dip is less than 90˚ it might make a difference which difference would be a maximum where the dip is 0˚ & is of the following kind. Suppose a pendulum swing at the magnetic equator and in the direction of the magnetic meridian it would intersect no lines of magnetic force, but travel to & fro along them; & no electric currents would be induced in it, no magneto-electric state obtain, & no resistance to motion, of the kind spoken off [sic], be produced:- but if it were vibrated in a plane perpendicular to the magnetic meridian currents from the lower to the upper edge or from the upper to the lower would tend to be produced; and because the two edges move with different velocity would probably be produced. Whether such currents would be sensible in their effects there I cannot say - [(]it is hardly probable that they would be here where the dip is so considerable), even to the pendulum test.

As all these results depend upon the earths force & its direction, I do not think it likely that any supposed change from time to time in the currents of the earth are likely to affect a magnet irregularly, provided they are so small in amount as not to affect the magnet needle. Whatever leaves that untouched would not, I think, affect the pendulum.

I am My dear Sir | Your most truly | M. Faraday

G.B. Airy Esq | &c &c &c

Of course I speak only of what is known[.] Unknown results may remain for us to discover[.] MF

Address: Geo. B. Airy Esq | &c &c &c | Royal Observatory | Greenwich