| Show NEW electrical tests with a specially devised electrical made by interior department investigators at the pittsburgh experiment peri ment station of the bureau of mines resulted in doubling the range of detection of earth sounds attained by the original unimproved the instrument experimented me anted with by the bureau of mines embodies the adoption of the vacuum tube audio frequency amplifier to the by this means the sound waves caught by the are converted into electrical waves in field tests made at the bureaus experimental coal mine at bruceton pa the original type and the improved electrical were used when blows were struck on a coal rib of parallel entry the improved electrical detected the sounds through feet of earth rock and coal whereas the original could detect such blows at no greater distance than feet the principle of the in the detection of earth sounds is that it transforms an earth wave into an air wave or pulsation which is heard by the ear as sound and at the same time magnifies the wave so that the sound is louder than if the ears were placed in direct contact with the earth two geo phones are generally used at one time the is limited in sensitivity however and any improvement in the theory and mechanism of the appliance would widen its field of usefulness |
