To James Alexander
Printed in Benjamin Franklin, Experiments and Observations on Electricity (London, 1769), pp. 282–3.
Suppose a tube of any length open at both ends, and containing a moveable wire of just the same length, that fills its bore. If I attempt to introduce the end of another wire into the same tube, it must be done by pushing forward the wire it already contains; and the instant I press and move one end of that wire, the other end is also moved; and in introducing one inch of the same wire, I extrude, at the same time, an inch of the first, from the other end of the tube.
If the tube be filled with water, and I inject an additional inch of water at one end, I force out an equal quantity at the other, in the very same instant.
And the water forced out at one end of the tube is not the very same water that was forced in at the other end at the same time, it was only in motion at the same time.
The long wire made use of in the experiment to discover the velocity of the electric fluid, is itself filled with what we call its natural quantity of that fluid, before the hook of the Leyden bottle is applied to one end of it.
The outside of the bottle being at the time of such application, in contact with the other end of the wire; the whole quantity of electric fluid contained in the wire is, probably, put in motion at once.
For at the instant the hook, connected with the inside of the bottle, gives out; the coating, or outside of the bottle, draws in a portion of that fluid.
If such long wire contains precisely the quantity that the outside of the bottle demands, the whole will move out of the wire to the outside of the bottle, and the over quantity which the inside of the bottle contained, being exactly equal, will flow into the wire, and remain there, in the place of the quantity the wire had just parted with to the outside of the bottle.
But if the wire be so long as that one-tenth (suppose) of its natural quantity is sufficient to supply what the outside of the bottle demands, in such case the outside will only receive what is contained in one-tenth of the wire’s length, from the end next to it; though the whole will move so as to make room at the other end for an equal quantity issuing, at the same time, from the inside of the bottle.
So that this experiment only shews the extream facility with which the electric fluid moves in metal; it can never determine the velocity.
And, therefore, the proposed experiment (though well imagined, and very ingenious) of sending the spark round through a vast length of space, by the waters of Susquehannah, or Potowmack, and Ohio, would not afford the satisfaction desired, though we could be sure that the motion of the electric fluid would be in that tract, and not under ground in the wet earth by the shortest way.
4. Printed directly following Alexander’s letter (reprinted immediately above) this undated document bears the caption “Answer to the foregoing; by B.F.”