The D'Arsonval galvanometer is a moving coil ammeter. The moving coil galvanometer is practically immune to ambient magnetic fields. A tangent galvanometer is an early measuring instrument used for the measurement of electric current. When an electric current flows through the galvanometer coil, a second magnetic field is created. The variable resistor is adjusted until the galvanometer reads zero. Resonant galvanometer scanners are used for this application. Stewart measured radiated power with a thermo-pile and sensitive galvanometer read with a microscope. A left side viewfinder galvanometer needle pointer indicated the exposure on a shutter speed scale. The currents were measured by a sensitive galvanometer. This induces a current in the coil which can be registered on the galvanometer. A Faraday disk can also be operated with neither a galvanometer nor a return path. It consists of a housing that contains lasers, mirrors, galvanometer scanners, and other optical components. A ballistic galvanometer is a type of sensitive galvanometer; commonly a mirror galvanometer. An early mirror galvanometer was invented in 1826 by Johann Christian Poggendorff. He coined the term "mirror galvanometer" and patented the device in 1858.