JV. The J V is an attempt to develop an amplified induction mine unit. JV = Jnduction Verstärker (Induction Amplifier). This was attempted purely as a result of the shortage of critical materials, nickel and copper. Two types were experimented with. One was a plain induction unit for use in the LMB mine, and the other for use in combination with a unit component operating on another influence in the LMB mine. The coil was wound on a soft-iron core one inch in diameter and 24 inches long, and consisted of two coils in ser-ies totalling 60,000 turns of 0.03-mm copper wire. In the JV unit the medium and high frequencies are by-passed by an additional unit. The circuit is designed to respond only to very low frequencies (approximately 1/30 to 1/50 cps.). Some experimentation was alos carried on with a similar three-stage amplifier, arranged in such a way that, after the induction system had been ampplified and operated, it would be switched out and the amplifier used as a low-frequency amplifier ("Klotzverstärker") for a low-frequency detector. Devolopment work on this system was stopped, as it was considered too diffi-cult a problem to work out a satisfactory protection system against nearby detonations.