An early commercial ECG machine, built in 1911 by the Cambridge Scientific Instrument Company A<unk>  is a sensitive fast-responding measuring instrument that uses a single fine filament of wire suspended in a<unk>  magnetic field to measure small currents. In use, a<unk>  light source is used to illuminate the fine filament, and the optical system magnifies the movement of the filament allowing it to be observed or recorded by photography. The principle of the<unk>  remained in use for electrocardiograms until the advent of electronic vacuum-tube amplifiers in the 1920s.
String galvanometer
Microperimetry
Four prism dioptre reflex test
Pelvimetry
Dosimetry
Urinometer
Keratometer
Vernier acuity