"Pressure" and voltage

[gar]

130914-1031 EDT

In Greenfield Village in the reduced scale replica of the Edison Illuminating Company's Station A (Detroit where Henry Ford was chief engineer in the 1890s) on a wall is an instruction sheet for operation of an Edison power plant. I don't believe this set of instructions relates to this plant in the 1890s.

Referenced in the instructions is the word "Pressure". Upon study it becomes clear that pressure is the word used for voltage prior to the mid 1880s. Dictionary.com indicates the origin of voltage as 1885-1890. While http://en.wikipedia.org/wiki/Volt presents the following:

In the 1880s, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt as the unit for electromotive force. At that time, the volt was defined as the potential difference [i.e., what is nowadays called the "voltage (difference)"] across a conductor when a current of one ampere dissipates one watt of power.

and

The international volt was defined in 1893 as 1/1.434 of the emf of a Clark cell. This definition was abandoned in 1908 in favor of a definition based on the international ohm and international ampere until the entire set of "reproducible units" was abandoned in 1948.

Therefore it appears that pressure was the description for electromotive force prior to about 1885, and that the set of instructions was for an early Edison power plant. Quite possibly the Peral Street Station 1882. Jumbo Number 9 dynamo is on display in this replica power house. Number 9 was a direct drive from the steam engine.

Also note that the Weston d'Arsonval meter did not exist in 1882. In the set of instructions mentioned above it appears that a galvanometer in some sort of null balance mode was used for "pressure" measurement. This would imply a potentiometer and a standard cell, but there was no mention of such details.

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[al hildenbrand]

Interesting read, Gar. Thanks.

It is truly amazing how "young" our electrical understanding is, compared to the other trades, and also, how recent the jargon creation for it has been.