Capping the Jars -- Part 10 of Reconstructing the Kilbourne &Clark No. 6 Spark Kit
By Bart Lee, K6VK, for CHRS, 16 V ’19
. The Jars o’ Capacitance are now complete. Each of the glass jars – Leyden jars – now has an aluminum foil layer inside and another outside. The outside is grounded, or will be system-grounded. The jar tops, dropping lamp-chains to the inner foil, are connected to the clip on the left. The tops of the jars, when charged by the induction coil, will offer high voltage to the spark gap. As already noted, high voltage even in this primitive configuration, is dangerous; as Hugo Gernsbach warned for the customers of his Electro-Importing Company:
. “AIl our jars can be charged with even our 1/2-inch coil and the discharging crash of even our 1 — pint jar can be heard for blocks. It is powerful enough to kill a cat with a single discharge.” (emphasis added)
. Five new safety features seem warranted. A plastic cover insulates each jar top. The (red) wires from the tops are now high-voltage insulated wire. The joint positive terminal on the left is double insulated. A high voltage cable comes off this terminal. And, a warning sign is affixed:
. John Staples, W6BM measured one similar but smaller “jar” last year:
. I performed a simple experiment to measure the capacitance of a simple Leyden jar
. Using a peanut butter jar, I put aluminum foil inside and outside and measured the capacitance at 135 pF with the inside foil as close to the glass as reasonable, but not exactly.
. Filling the jar with salt water increased the capacitance to 470 pF, with a dissipation factor of 0.025. This means that the ESR measured at 1 kHz is about 8.5K, not a very good capacitor.
. The array of jars – dry – measures about 913 pF (picofarads – see photo).
. This is about 0.001 microfarad. Adding salt water (heavily salted back in that day) should multiply the capacitance by at least a factor of three.
. The term “Jar” as a measure comes from Marconi’s early gear. His Lizard Peninsula (Cornwall, UK) station circa 1910 is preserved (David Barlow, G3PLE, Curator), including its jars:
See: Transmitter Capacitance (“jars”) at Lizard Wireless Station (Lee photo).
. The Internet asks (and answers): “How many microfarad in 1 jar? The answer is 0.00111265.” So the new array, dry, is about one jar of capacitance. (The Royal Navy used the “jar” as a unit of capacitance until after World War One).
. A next step will be charging the jars. (de K6VK) ##