George Biddell Airy to Faraday   7 February 1855

Royal Observatory Greenwich | 1855 February 7

My dear Sir

We have observed on two occasions (under circumstances which appear to be general) an odd galvanic phenomenon, on which I wish to consult you.

For dropping the Time Signal Ball at Deal¹, there is a wire which, for about one minute of time before the ball-drop, is made continuous from Greenwich to Deal. At Deal it has connexion with the earth, but at Greenwich it has no connexion, as it is waiting to be joined to the battery at the proper instant. There are galvanometers at London and Tonbridge: and the ball-apparatus at Deal may be considered as a galvanometer of a delicate kind, for a very trifling current there drops the ball. The general direction of the line is very nearly west to east: deviating perhaps 30˚ from the magnetic west to east

Now the phenomenon is this. As soon as the contacts are completed so as to make the line continuous (not at Greenwich), the galvanometers at London and Tonbridge are deflected considerably (30˚), but there is no sign of current at Deal.

Can you explain this? Will it always be so?

The direction of deflection is the same as that produced by a current from the copper pole of a Greenwich battery.

As soon as the Greenwich battery contact is completed from copper pole, the deflection is normal at London and Tonbridge and there is abundant current at Deal.

Almost immediately after the ball drop, Greenwich is connected with earth, and then the deflexions at London and Tonbridge cease entirely.

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My assistant, now at Deal, finds that a battery nearly equal in strength to the dilute sulphuric acid battery, may be made with sea water. Can we rely on this?

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Possibly Mr. Barlow will shew you some criticism of mine on a paper of yours².

I am, my dear Sir, | Yours very truly | G. B. Airy

Professor Faraday