rfd-sense #4: "To investigate the condition or power of, as of steam engine, by means of an indicator." I believe "investigate" is wrong here, and the correct verb could be e.g. "to show", which would make this a specific case of sense #1. Another interpretation is that this is a definition of some other verb, such as "to inspect". --Hekaheka (talk) 04:27, 11 September 2012 (UTC)
That is the Webster 1913 definition, verbatim. Could be just call it dated, if it were cited? Is an indicator perhaps a specific device?
Eureka. Century has this encyclopedic definition of such an indicator: A steam (cylinder) pressure-gage. It is an apparatus for recording the variations of pressure or vacuum in the cylinder of a steam-engine. The accompanying cut represents a type of the indicator. The pipe with the stop-cock is screwed to the cylinder so that when the cock is opened the pressure of the steam within may enter the cylinder above, press the piston upward against the action of a spring constructed to give a definite resistance in pounds per square inch, and cause the lever-arm to rise and mark on the hollow cylinder at the left a vertical trace, the altitude of which measures the pressure. A card or a sheet of paper may be fitted to this cylinder, and the trace be made on the paper. The hollow cylinder is free to revolve, if drawn by the loose cord hanging from it. To operate the indicator, the cord is connected with some moving part of the engine so that a single stroke of the piston causes the cylinder and the card to revolve once (the return being secured by a spring) as the pencil makes one mark. Since the pencil-mark is timed to one stroke of the engine, the resulting curved line on the card gives a graphic report of the pressure or vacuum of the steam during one complete stroke. Such graphic curves are called indicator-diagrams, the marked card being called an indicator-card. See indicator-diagram, under diagram. DCDuringTALK 05:20, 11 September 2012 (UTC)
Wow! Perhaps the sense should be tagged somehow to indicate that this is a very specific usage. --Hekaheka (talk) 05:55, 11 September 2012 (UTC)
I believe these are citations:
1903, "How to indicate an engine" in The Star Improved Steam Engine Indicator, page 64:
To a person who is familiar with the use of an indicator, whether it be of one make or another, it is needless to give instructions as to how an engine should be indicated, [...]
1905, Power, volume 25, page 448
I found it fully as easy to indicate an engine at a speed of 320 to 340 revolutions as at 80.
1905, Central station, volume 5, page 76
An indicator will give the working of these valves at all times and soon return its cost in higher engine efficiency. The day has passed when it was only the expert who could indicate an engine or afford to own an indicator.
Oddly, all the citations I could find fell in the very narrow band of 1901 to 1905. I wonder if it was a piece of engineering jargon that just never really caught on (or a strange hole in Google's collection). Smurrayinchester (talk) 15:48, 11 September 2012 (UTC)
Right around preparation time for Century and MW 1913. Webster 2nd c. 1934 expanded their 1913 def of indicator imitating and possibly surpassing Century's def, so it still had some currency then. MW online has a much reduced definition in the same sense. But now it seems as quaint as telex, twx, modem, parallel printer port are and carburetor and fax soon will be. DCDuringTALK 16:17, 11 September 2012 (UTC)
What seems to have happened is that "indicating" became a much less skilled task, with the improvement of the instruments and the dissemination of knowledge about reading the "indicator cards" and charts and other types of diagnostic and control instruments (indicators). There is plenty of discussion in the 21st century about "indicated pressure" in discussions of turbines and internal-combustion engines. DCDuringTALK 16:34, 11 September 2012 (UTC)
Keep. It's real, it's cited, and it's not covered by any other sense. Equinox◑ 00:53, 16 September 2012 (UTC)