para quem quer saber um pouco mais sobre o assunto.
Solenoid chronograph -This system depends on the principle that when a magnet is thrust into a coil of wire an e.m.f.(electronic magnetic field), is set up and, if the coil be part of a closed circuit, a current will flow. The wired screens of the Boulengé system are replaced by frames, each wound with a coil of wire. The coils are connected in series with each other and with the sensitive element of an oscillograph. The projectile is magnetized by holding it in the field of a fixed coil with a heavy current flowing for a few seconds immediately before loading. The passage of the projectile through each screen sets up a current whose occurrence is recorded by the oscillograph.
Valls
PS: e para quem quiser se aprofundar, tem este artigo sobre o Naval Proving Ground.
