137. Portable Mine Detector Set (Minierhorchgerät)

a. Description. The portable mine-detector set is intended to provide a simple means of detecting tunneling activity. It weighs only 35 pounds. Each set con-sists of 10 detector heads, 10 earth spikes, 1 terminal unit with 1 head-set, 2 dry cells, and 1 carrying case.

(1)

Detector heads. Each detector head consists of a carbon microphons which has good low-frequency response. A damped oscillation pendulum, having a frequency of 11 cycles per second, is in contact with the carbon diaphragm. The pendulum is held and away from the diaphragm by a simple spring-loaded plunger until the mic-rophone is placed in position on its ground spike. A spirit level is incorporated in the top of the microphone case to assure that the pendulum is vertical.

(2)

Ground spike. The ground spike is a light-metal, corkscrew type spike, 11 3/4 in-ches long. A bar is provided for screwing the spike into the earth. A locking univer-sal joint fits over the end to receive the microphone.

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Terminal unit. The teminal unit consists of a 10-pair terminal strip with a 10-position switch by means of which the headset may be connected to any desired detector head. One terminal of each of the switches of the 10 pairs of detector-head cables is a common terminal. The terminal units is mounted in the lid of the standard Ger-man field telephone case, while the dry cells and the headset are carried in the lower half of the case. In some cases a plug and jack arrangement is provided for connecting the microphones in parallel groups of three for preliminary separation of sound location before reverting to use of the 10-position switch.

b. Employment. This set may be used in conjunction with the mulitple ignition set 44 (par. 149) for detonating controlled mine fields.