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I'm aware of the general origin of "a head of steam" being from old steam-powered trains, but how does it break down? I'm guessing that the "head" is the surplus of motive force necessary to break free from a standstill; does some older definition better meet that meaning? The closest seems to be

a body of water kept in reserve at a height; also : the containing bank, dam, or wall

Is there a more direct fit or does it rely on analogy?

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  • 4 answers, all by geeks explaining the engineering of such ... no etymology!
    – lbf
    Jun 28 '18 at 15:19
  • Head refers to the end of the pressure vessel, called a head (which is where ICEs got the term head from). And it's a nice short word to use as a gauge label to distinguish it from jacket pressure, manifold pressure, bleed pressure, etc.
    – Phil Sweet
    Jun 28 '18 at 16:43
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No analogy. It's an engineering term. Head is a measure of pressure. Head is measured in units of distance, (typically feet in the US) and is the height of a column of fluid (usually water) that exerts an equivalent pressure at the bottom of the column.

For example here is a table of head vs psi: enter image description here

So when the engineer wants to get the train moving he needs to build up enough 'head' (steam pressure) to move the train.

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  • Interesting. Any idea why they picked 'head' for that term? Seems like an odd word choice.
    – Lynn
    Mar 16 '12 at 1:24
  • I don't know for certain but I surmise it comes from measuring pressure at a wellhead.
    – Jim
    Mar 16 '12 at 2:36
  • @Lynn,Jim: I don't know any more than you guys, but anatomically for humans the distance from your "head" to the ground is the same thing as "height". And I note this Wiktionary Talk lumps "head of pressure" in with "crisis coming to a head" as related "meanings". But I'm inclined to think wellhead is a somewhat different sense - more akin to, say, bulkhead. Mar 16 '12 at 4:24
  • @FumbleFingers you may be right. My thinking was that head as in the head of a stream or river is where it originates and comes out of the ground, the wellhead is where the water (or oil) comes out of the ground. The pressure is typically highest at the head and reduces after it leaves the mouth. So I can easily see people asking how much "head" they've got when asking about the pressure at the wellhead. But as I say, it's purely conjecture at this point.
    – Jim
    Mar 16 '12 at 5:55
  • @Jim: I'm not an engineer, but my understanding of physics is the amount of head you have relative to the wellhead depends on how much higher or lower you are than that point. Basically, height is always what counts, whatever it's relative to. Mar 16 '12 at 6:09
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I am an engineer. "Head" is a a term of pressure. There are components to it though. There is static head, pressure head, and dynamic head. A head of steam refers mainly to pressure head. This is the pressure of steam in the steam drum of a pressure (usually propulsion for trains or steam ships) boiler after it has been fired to anything above atmospheric pressure, and thus has the ability to do work. There are other things called "friction head", "dynamic head" and "static head" that are also common commonly measured in terms of feet. The "feet" means the feet of height of a column of liquid of the same temperature of the fluid whose pressure is being measured. Head is used extensively in engineering terms when specifying boilers, pumps, etc, and also in the design of piping systems which need to account for "head loss" due to pipe friction from the fluid moving inthe pipes. Old steamship engineers like myself used the term occasionally for one main purpose and that was when raising steam pressure to it's design pressure for the boiler before using it to roll the turbines. And I am not that old, so maybe that explains why we never really used that term to much. We had a big old pressure gauge that read in PSIG that told us what we needed to know.

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As other answers point out, this is a term to describe pressure. The most basic technique for creating water pressure involves either storing the water in a raised container (think water towers) or in maintaining a reservoir of sufficient depth to achieve relative height. The higher the height, called "head," the higher the pressure, which is why pressure formulated as "head" is measured in distance (often feet).

The use of the term "head" is appropriate simply because it is a measure of height, and the highest point of anything can be referred to as a head.

Although steam is a different method of generating pressure, the term carries over. As you can see from the chart in Jim's answer, although in the phrase you're asking about the use is colloquial, not technical, it is not just a figure of speech put a defined unit of measurement as well.

Reference:

A Text Book on Hydraulics - 1906

Section 61 (at the bottom of the linked page):

61. Meaning of "Head" - The word head is used by writers on Hydraulics in a somewhat indefinite way. In all cases it means the height of a column of water, either actual or ideal. Thus, "head on an orifice," or on any point of an orifice, has in preceding discussions been used to designate the vertical height of the free surface of water above the point under consideration.

Merriam-Webster, entry "Head"

Definition 9a: the uppermost extremity or projecting part of an object

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Head, as in cylinder head. A steam piston and cylinder has a head. The end of the cylinder, generally bolted on. A full head of steam referres to a cylinder full of high pressure steam, ready to do work (move the piston) and drive the train forward.

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  • 1
    this is incorrect. A steam engine can work up a full head of steam without out releasing any of it to the cylinders. See Eric’s answer.
    – Jim
    May 22 '18 at 6:39

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