On the other hand, for a crystal radio with low-impedance phones, it's a low resistance microammeter connected in series with the phones. That is, a sensitive ammeter that does not drop headphone current signnificantly. A voltmeter was discovered in my junkbox to build such a meter for my crystal radio which uses low-impedance balanced-armature phones.
On removing its series and shunt resistors, the resistance of the 1mA FSD movement turned out to be only 60 Ω and hence quite suitable for the purpose. The existing graduations on the meter face were removed and replaced with graduations 0 -10 using a lead pencil.
|Crystal Radio S-Meter - Schematic|
The meter was already mounted on a square bakelite flange. The flange mounting holes served as terminal mounting holes, obviating the need for an enclosure.
|Crystal Radio S-Meter|