This demagnetisation he detected by means of the movement of the needle of a magnetometer placed near one end of the iron wire.
"Hertzian Wave Wireless Telegraphy"
John Ambrose Fleming
The intensity of this induced magnetisation may be measured by noting the deflection it produces on a freely suspended magnet in a magnetometer.
"Response in the Living and Non-Living"
Jagadis Chunder Bose
But another method of detecting the position of a metallic mass is by the use of the magnetometer.
"The Journal of Submarine Commander von Forstner"
Georg-Günther von Forstner Commentator: John Hays, Jr. Hammond