(n.) An instrument for writing in the dark, or without seeing.

ECG EEG EKG X ray X-ray movie cable cablegram chest X-ray day letter electrocardiogram electrocorticogram electroencephalogram electroencephalograph electromyogram encephalogram encephalograph fast telegram night letter orthodiagram photofluorogram photofluorograph photogram pyelogram radiograph roentgenogram roentgenograph scotograph shadowgram shadowgraph skiagram skiagraph telegram telegraph telex wire