Capacitance Miscellany

25 December,1750

Dear Friend

I have lately made an experiment that I desire never to repeat. Two nights ago, being about to kill a turkey by the shock from two large glass jars, containing as much electrical fire as forty common phials, I inadvertently took the whole through my own arms and body, by receiving the fire from the united top wires with one hand while the other held a chain connected with the outside of both jars. The company present (whose talking to me and to one another, I suppose, occasioned my inattention to what I was about) say that the flash was very great, and the crack as loud as a pistol, yet, my senses being instantly gone, I neither saw the one nor heard the other; nor did I feel the stroke in my hand, though afterwards found it raised a round swelling where the fire entered, as big as half a pistol bullet, by which you may judge the quickness of the electrical fire, which by this instance seems to be greater than that of sound, light, or animal sensation.

What I can remember of the matter is that I was about to try whether the bottles or jars were fully charged by the strength and length of the stream issuing to my hand, as I commonly used to do, and which I might safely enough have done if I had not held the chain in the other hand. I then felt what I know not how well to describe - a universal blow throughout my whole body from head to foot, which seemed within as well as without; after which the first thing I took notice of was a violent, quick shaking of my body, which gradually remitting, my sense as gradually returned, and then I thought the bottles must be discharged, but could not conceive how, till at last I perceived the chain in my hand, and recollected what I had been about to do.

That part of my hand and fingers which held the chain was left white, as though the blood had been driven out, and remained so eight or ten minutes after, feeling like dead flesh; and I had a numbness in my arms and back of my neck, which continued till the next morning, but wore off. Nothing remains now of the shock but a soreness in my breastbone, which feels as if it had been bruised. I did not fall, but suppose I should have been knocked down if I had received the stroke in my head. The whole was over in a minute.

You may communicate this to Mr Bowdoin as a caution to him, but do not make it more public, for I am ashamed to have been guilty of so notorious a blunder; a match for that of the Irishman whom my sister told me of, who, to divert his wife, poured the bottle of gun powder on the live coal; or that of the other, who, being about to steal powder, made a hole in the cask with a hot iron. I am yours,etc,

B.Franklin

From the Admiralty Handbook of Wireless Telegraphy 1931.
The jar is the Service unit, and is very useful when dealing with the small capacities met with in ordinary wireless practice.
1 farad = 9x108 (nine hundred million) jars
1 microfarad = 900 jars
[It was a big jar, 10 metres in radius, more like a balloon. By 1937 this unit was made obsolete or at any rate obsolescent to bring Royal Navy practice into line with commercial.