Q: Why is it a “speedometer,” not a “speedmeter”? That thing on the side of my house is a “gas meter,” not a “gasometer,” and the electrician has an “ohm meter,” not an “ohmometer.”
A: The letter “o” appears frequently as a connective or linking element in English compounds where at least one of the parts is of Greek origin.
The English construction can be traced back to the use of the omicron (o) at the end of the first part of a compound in classical Greek. For example, δημο-κρατία (demo-cratia, rule of the people), ϕιλο-σοϕία (philo-sophia, love of knowledge), and νεκρo-πολις (necro-polis, city of the dead).
In ancient Greek, nouns that ended with an omicron and a sigma (-ος, or os in the Latin alphabet) formed compounds by dropping the sigma and keeping the omicron as a connective. Classical Latin used the letter o similarly in compounds borrowed from Greek as well as some that originated in Latin. Later, French and the other romance languages inherited these compounds from classical or medieval Latin. English, in turn, adopted many of them from French or Latin.
Although the omicron in classical Greek was often the final letter in the first part of a compound, it’s frequently treated in modern English as the first letter in the last part of a compound, especially if the first part is a native English word that ends in a consonant.
As the Oxford English Dictionary explains, the connective “-o-” in some compounds “tends to be treated as if it were part of the termination, particularly where the latter is combined with an English first element which ends in a consonant.”
The term “speedometer” is a good example of this. It’s a compound made up of the noun “speed,” which dates back to Anglo-Saxon times, the connective “-o-,” and the combining form “-meter,” which comes from the Greek -μέτρον (-metron, or measure).
However, there are a lot of exceptions, as you’ve noticed. Many standard dictionaries, for example, have entries for both “gas meter” and “gasometer,” though the two terms have different meanings. A “gas meter” is a device for measuring the amount of gas used at a property, while a “gasometer” is a tank for storing and measuring gas.
The measuring devices named after the German physicist Georg Ohm and the Italian physicist Alessandro Volta have been written several different ways over the years—as two words, hyphenated, and as one word, sometimes with the connective “-o-” and sometimes without it. Most standard dictionaries now list the device for measuring electrical resistance as an “ohmmeter” and the device for measuring electrical potential as a “voltmeter.”
Interestingly, we’ve seen the two-word term “speed meter” used once in a while in writing to refer to the radar and laser devices used by police to catch speeders, though the usage hasn’t made its way into standard dictionaries.
The OED, an etymological dictionary based on historical evidence, has separate entries for the combining forms “-meter” and “-ometer” as well as for the “-o-” connective. The connective entry treats “-ometer” as a two-part term made up of “-o-” and “-meter.”
The “-ometer” version was the first to show up in English. As the OED explains, “Words containing this terminal element are first attested in English in the 17th cent., the earliest significant example being thermometer n., modelled on the earlier French thermomètre; the next is barometer n., an English formation (French baromètre is recorded almost contemporaneously).”
“In the early formations the ending is always appended to Greek noun stems or combining forms [ending] in -o,” the dictionary says, but during the 18th century “formations begin to appear in which the initial element could be of Latin or other origin.”
The earliest example with an English initial element, according to the OED, “is the humorous word passionometer n. (mid 18th cent.); this is succeeded in the 19th cent. by a small number of similar rarely-used humorous words, e.g. foolometer n., obscenometer n.”
“Speedometer,” the word you asked about, appeared in the early 20th century. The first OED citation is from the Aug. 4, 1904, issue of the Times (London): “His ‘speedometer’ … showed he was going at only ten miles an hour.”
Getting back to your question, there’s no definite reason why the instrument that measures the speed of a vehicle is a “speedometer” while the device that measures the use of gas in a house is a “gas meter.”
As we’ve said in other posts, the development of English has not been tidy. We’re reminded of that nearly every day as we translate Old English and Middle English into Modern English.
Take the noun “speed” for example. In Old English (spoken from around 450 to 1150), the noun was spelled spoed or sped. In Middle English (roughly 1150-1500), it was spede, speede, spied, speid, spyd, spyde, speed, and so on. Not until the 17th century did “speed” emerge from the pack and become the dominant spelling—a process that wasn’t too speedy.