Nowadays we buy capacitors in farads, μF, and pF. But in the early years of radio, capacitances were often stated in jars.
From the (British) 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 = nine hundred million jars
1 μF = 900 jars
Variable capacitors were difficult to make in the early 20th century, and transmitters were tuned by adding or removing fixed capacitors in parallel. Almost always, each of these capacitors was a Leyden jar and actually consisted of a thin-walled glass jar with metal (foil or plating) on the inside and outside surfaces. Operators would add or remove capacitance by hooking and unhooking connections to the jars, and often prepared tables listing how many Leyden jars to use for each operating frequency.
Soon the Leyden jars became more or less standardized, and a typical one had a capacitance of about 1111 pF. This value of capacitance came to be called a jar, and the Royal Navy used this as a standard unit until 1937, when it was made obsolescent in order to align Royal Navy practice with commercial practice. Some of the fleet may have used the jar as a unit of measurement as late as the early 1950s.
I believe, without being certain, that the US. Navy also specified capacitance in jars in the early 20th century.
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-- Carl
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