A cardiogram is a record of muscle activity, and a cardiograph is the machine that produces it — I know and understand this difference. That said, every time I’m having to use one of these words (or recollect them), I confuse the two. Is there a good way to remember which is which?
This really is tricky because there is no hard and fast rule.
Autographs do not make autograms; monographs do not make mongrams, and photographs do not make photograms. In fact:
A cardio–gram is a record of muscle activity. A cardio-graph makes cardiograms.
An auto-gram is a sentence that describes itself in the sense of providing an inventory of its own characters. An auto–graph is a signature.
A photo-gram is a picture produced with photographic materials, such as light-sensitive paper, but without a camera. A photo-graph is an image created by light falling on a photosensitive surface, usually photographic film or an electronic image sensor, such as a CCD.
A mono-gram is a motif made by overlapping or combining two or more letters or other graphemes to form one symbol. A mono-graph is a specialist work of writing on a single subject or an aspect of a subject, often by a single author, and usually on a scholarly subject.
If it is only the ‘cardio-‘ pairing that is important then I might remember by saying that the one with the m (i.e. cardiogram) is the one that is NOT the m-achine. But that’s just the way my mind works!