Geiger counter

Geiger in Geiger counter is a surname and should be capitalized. Write Geiger counter (two words), not geiger counter.

Below: a short rule, clear examples you can copy for work, school, or casual writing, and quick rewrites to fix common mistakes fast.

Quick answer

Yes - capitalize Geiger in Geiger counter because it's an eponym. Write Geiger counter (capitalize the name; counter stays lowercase in running text).

Treat Geiger like any other proper name: Geiger counter, not geiger counter.
No hyphen in normal use: Geiger counter. Use Geiger-Müller counter (or Geiger-Muller) when both names appear.
Plurals and possessives follow normal rules: Geiger counters, the Geiger counter's display.

Core explanation

When a device is named for a person, capitalize the person's name and leave the common noun lowercase. If you would capitalize an obvious name substitution (Einstein) in the same spot, capitalize Geiger.

Pattern: Proper-name + common-noun → capitalize only the name (Geiger counter).
Start of sentence: Capitalize normally (Geiger counter is stored in the cabinet.).
Referring to the inventor: Hans Geiger (capitalize the full name).

Grammar rules that apply

Apply standard capitalization and inflection rules to the name portion as you would to any surname.

Plural: Geiger counters (only the surname is capitalized).
Possessive: the Geiger counter's probe (Geiger's notes when referring to the person).
Title/headline case: always capitalize Geiger; style guides differ on Counter.

Example - wrong: the geiger counters need inspection.Right: The Geiger counters need inspection.
Example - wrong: geiger counter's battery was dead.Right: The Geiger counter's battery was dead.

Hyphenation and spacing

Standard form is two words: Geiger counter. Avoid hyphenation unless a compound modifier is unavoidable; rephrase instead.

Use Geiger counter (no hyphen).
When both names appear historically, keep capitals and diacritics: Geiger-Müller counter (or Geiger-Muller counter).
Prefer rephrasing for clarity: reading from the Geiger counter rather than Geiger-counter reading.

Wrong: He left a Geiger-counter on the bench.
Right: He left a Geiger counter on the bench.
Wrong: we use a geiger-muller counter in class.
Right: We use a Geiger-Müller counter in class.

Real usage: work, school, casual

Use these lines verbatim or adapt them for emails, lab notes, memos, homework, or chats. Be consistent across a document and follow your organization's headline style for the word counter.

Work: We calibrated the Geiger counter before recording the background radiation levels.
Work: All field technicians must carry a Geiger counter and log readings each hour.
Work: Inventory: Four Geiger counters assigned to Building B.
School: Use a Geiger counter to measure counts per minute at three distances.
School: Hans Geiger co-developed the detector later called the Geiger counter.
School: Explain why the Geiger counter reading increases near the sample.
Casual: I found an old Geiger counter at a flea market - works surprisingly well!
Casual: My Geiger counter spikes when I point it at the ceramic - any ideas?
Casual: Took a Geiger counter to the vintage watch fair; background was normal.

Try your own sentence

Test the full sentence rather than the phrase alone. Context usually makes capitalization clear.

Examples: common wrong/right pairs

Wrong examples show typical typing; right examples show the corrected form.

Wrong: do you own a geiger counter for your experiments?
Right: Do you own a Geiger counter for your experiments?
Wrong: the geiger counter readings jumped after the spill.
Right: The Geiger counter readings jumped after the spill.
Wrong: she bought a cheap geiger counter online.
Right: She bought a cheap Geiger counter online.
Wrong: turn on the geiger counter before entering the lab.
Right: Turn on the Geiger counter before entering the lab.
Wrong: we replaced several geiger counters last month.
Right: We replaced several Geiger counters last month.
Wrong: inventory: 2 geiger counters, 3 probes.
Right: Inventory: 2 Geiger counters, 3 probes.
Wrong: the geiger counter's display is cracked.
Right: The Geiger counter's display is cracked.
Wrong: we demonstrated a geiger-muller counter in class.
Right: We demonstrated a Geiger-Müller counter in class.
Wrong: geiger counter operators must wear PPE.
Right: Geiger counter operators must wear PPE.

Rewrite help: fix your sentence in 3 quick steps

Three quick checks: (1) Is the first word a name? Capitalize it. (2) Keep the following noun lowercase unless it's also a name. (3) Rephrase compound modifiers instead of hyphenating.

Step 1: Identify the name → capitalize it (Geiger).
Step 2: Ensure the common noun is lowercased (counter).
Step 3: Reword awkward modifiers: reading from the Geiger counter.

Rewrite:
Original: the geiger counter was left on the table.
Rewrite: The Geiger counter was left on the table.
Rewrite:
Original: our geiger counter-reading showed a spike.
Rewrite: Our reading from the Geiger counter showed a spike.
Rewrite:
Original: geiger counter operators must wear badges.
Rewrite: Geiger counter operators must wear badges.

Memory tricks and quick checks

Keep a tiny style note for high-frequency eponyms and use quick mental checks.

Mnemonic: If you'd write Einstein, write Geiger the same way.
Quick check: Substitute a well-known name (Einstein). If you'd capitalize it, capitalize Geiger.
Maintain a one-line list of eponyms you use often (Geiger counter, Foucault pendulum, Morse code).

Similar mistakes to watch for

Eponyms behave differently: some become common nouns and lose capitalization, others retain the surname. When in doubt, consult a dictionary or your style guide.

Units named after people are lowercase in scientific usage (newton, pascal) but refer to the person with a capitalized name.
Some eponyms become common nouns and are lowercased over time (diesel engine, sandwich).
Device names usually keep the surname capitalized (Foucault pendulum, Geiger counter).

Wrong: He checked the diesel Engine for leaks.
Right: He checked the diesel engine for leaks.
Wrong: The pascal is named after Blaise Pascal.
Right: The pascal is named after Blaise Pascal. (Unit names are lowercase in scientific usage.)
Wrong: we used a foucault Pendulum in the demo.
Right: We used a Foucault pendulum in the demo.

FAQ

Should I capitalize geiger counter in a headline?

Capitalize Geiger because it's a proper name. For counter, follow your headline style (title case vs sentence case), but never lowercase Geiger.

Is Geiger counter capitalized in technical journals?

Yes. Journals capitalize eponymous names like Geiger. Check the journal's house style, but use Geiger counter unless instructed otherwise.

What is the correct form for Geiger-Müller counter?

Use Geiger-Müller counter with both names capitalized. If you can't use diacritics, Geiger-Muller counter is acceptable; keep both capitals.

When do eponyms become lowercase (like diesel)?

Some eponyms become commonized and lose capitalization over time. Consult a dictionary or style guide for each term.

Quick ways to check a sentence for capitalization errors?

Substitute a well-known name (Einstein), run a grammar checker, or consult your organization's style guide. Keep a short personal list of frequently used eponyms.

Need a quick capitalization check?

Paste the sentence you're unsure about into a grammar tool or your editor and check whether the first word is an eponym. Small fixes like writing Geiger counter correctly make reports, papers, and messages look more professional.