Finding Land Mines by Sound
a short article from
illustration of a device that would become commonly used in later wars
FINDING BURIED SHELLS BY SOUND
A REMARKABLE IDEA.
When the battlefields of Europe are reclaimed for agriculture, there is an ever-present risk of death or serious injury to farmers and horses from ploughshares coming in contact with buried, unexploded shells.
The danger is regarded as particularly grave in certain parts of France, and recently an engineer was commissioned to devise a method for the discovery of such shells.
The instrument devised by the French is an adaptation of the Hughes induction balance. The original instrument was able to detect a small-calibre shell at a depth of about forty centimetres (nearly sixteen inches). The apparatus is so sensitive that its user can detect by the sounds in the head telephones the proximity of a mere scrap of shell on or near the surface, or even a tin cup.
Two coils of large diameter are employed, the device being two induction balances used as one. The diameter of the coils is about twenty-eight inches. The winding of the primary circuit consists of twenty layers, that of the secondary circuit ten layers, on wooden spools.
The two windings are placed one beside the other about one centimetre apart, so that the mutual induction of the primary and secondary circuits will be brought to the neutral state, resulting in the telephone-receivers remaining silent in the absence of any metallic mass in the vicinity. The desired sensibility is obtained by suitable telephones. The two receivers of the headpiece are connected, and the thickness of the diaphragm of each reduced to half that of conventional instruments. The primary. periodic current is produced by four dry cells and an electro-magnetic vibrator interrupter similar to those on medical coils. A key or a pocket-knife brought near the centre of one of the coils ought to provoke a perceptible sound.
To explore ground, the two coils each of which comprises a primary and secondary winding, are placed on two vertical sticks attached to the ends of a horizontal bamboo. An assistant carries the device, and walks holding the coils of the balance a few centimetres above the ground. The observer, wearing a telephone headpiece and carrying a box containing the battery, condenser, vibrator and regulating apparatus, follows a flexible conducting cable connecting the exploring coils with the apparatus of the observer. Fragments of shells, or tin cans or boxes, near the surface produce a sound as intense as that made by a deeply buried shell, but it is easy to distinguish between. A superficial object exerts the greatest influence when near the edges of the exploring coils. A projectile deeply buried, on the contrary, exerts its maximum action when under the centre of either coil, and gives only one reinforcement of the sound during the passage of the instrument.
Each coil explores a strip equal to its width; therefore two strips, each seventy centimetres wide, are examined during each passage. It requires about three hours for two persons to explore two and a half acres.