An electromagnetic oscillograph is an oscillograph which measures variations of electric current by having it go through a magnetic coil. Variations in current induce momentum in the coil, which can be directly measured. The electromagnetic oscillograph was invented by William Duddell. Some models utilise a mirror which reflects a beam of light, allowing measurement of minute movements of the coil. Other were fitted with a hand, possibly fitted with a pen to record values.
References
External links William du Bois Duddell Electromagnetic oscillograph