Measurement of the Capacitance between a Grapefruit and the Great Beyond

Donald Daniel

Oct 2015

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In my old age I was reflecting on the reckless adventures of my youth. I remembered when I measured the capacitance between a grapefruit and the great beyond. Lest the reader jump to the conclusion that this is nonsense, recall from elementary physics that there is not only the capacitance between two objects, but also the self capacitance of an object. This latter is the capacitance between the object and infinity, assuming that infinity is grounded. Infinity is also sometimes referred to as the great beyond. So I measured the self capacitance of a grapefruit.

Smythe [1, p.121] gives the capacitance between a conducting sphere and a grounded plane. The sphere does not have to get very far above the grounded plane before the capacitance is practically the same as the self capacitance of the sphere. So a measurement of the capacitance between a grapefruit and ground when the greapfruit is on an insulating shelf well above ground will do.

Hague and Foord [2] shows bridge technology. I did not take notes, and cannot remember the bridge I used. The leads were coaxial cable, with the center conductor protruding from the shielding where it needed to penetrate the grapfruit to contact the electrically conducting juice in the grapefruit. The protruding conductor was long enough that the shielding did not get too close to the grapefruit.

1. Smythe, William R. Static and Dynamic Electricity, 2nd ed. 1950 McGraw Hill.

2. Hague B. and Foord T.R. Alternating Current Bridge Methods, 6th edition, Pitman 1971.

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