Mass and Weight (Readers Advance Science Readers)
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French physicist and priest Jean de Hautefeuille built one in An early special-purpose seismometer consisted of a large, stationary pendulum , with a stylus on the bottom. As the earth started to move, the heavy mass of the pendulum had the inertia to stay still within the frame.
The result is that the stylus scratched a pattern corresponding with the Earth's movement. This type of strong-motion seismometer recorded upon a smoked glass glass with carbon soot. While not sensitive enough to detect distant earthquakes, this instrument could indicate the direction of the pressure waves and thus help find the epicenter of a local quake.
Such instruments were useful in the analysis of the San Francisco earthquake. Further analysis was performed in the s, using these early recordings, enabling a more precise determination of the initial fault break location in Marin county and its subsequent progression, mostly to the south. Later, professional suites of instruments for the worldwide standard seismographic network had one set of instruments tuned to oscillate at fifteen seconds, and the other at ninety seconds, each set measuring in three directions.
Amateurs or observatories with limited means tuned their smaller, less sensitive instruments to ten seconds. The basic damped horizontal pendulum seismometer swings like the gate of a fence. As the ground moves, the weight stays unmoving, swinging the "gate" on the hinge. The advantage of a horizontal pendulum is that it achieves very low frequencies of oscillation in a compact instrument.
The "gate" is slightly tilted, so the weight tends to slowly return to a central position. The pendulum is adjusted before the damping is installed to oscillate once per three seconds, or once per thirty seconds. The general-purpose instruments of small stations or amateurs usually oscillate once per ten seconds. A pan of oil is placed under the arm, and a small sheet of metal mounted on the underside of the arm drags in the oil to damp oscillations. The level of oil, position on the arm, and angle and size of sheet is adjusted until the damping is "critical", that is, almost having oscillation.
The hinge is very low friction, often torsion wires, so the only friction is the internal friction of the wire. Small seismographs with low proof masses are placed in a vacuum to reduce disturbances from air currents. Zollner described torsionally suspended horizontal pendulums as early as , but developed them for gravimetry rather than seismometry.
Early seismometers had an arrangement of levers on jeweled bearings, to scratch smoked glass or paper. Later, mirrors reflected a light beam to a direct-recording plate or roll of photographic paper. Briefly, some designs returned to mechanical movements to save money.
In mid-twentieth-century systems, the light was reflected to a pair of differential electronic photosensors called a photomultiplier. The voltage generated in the photomultiplier was used to drive galvanometers which had a small mirror mounted on the axis.
The moving reflected light beam would strike the surface of the turning drum, which was covered with photo-sensitive paper. The expense of developing photo sensitive paper caused many seismic observatories to switch to ink or thermal-sensitive paper. Modern instruments use electronic sensors, amplifiers, and recording devices.
Most are broadband covering a wide range of frequencies. The mechanical suspension for horizontal instruments remains the garden-gate described above. Vertical instruments use some kind of constant-force suspension, such as the LaCoste suspension. The LaCoste suspension uses a zero-length spring to provide a long period high sensitivity.
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Vertical and horizontal motions can be computed from the outputs of the three sensors. Seismometers unavoidably introduce some distortion into the signals they measure, but professionally designed systems have carefully characterized frequency transforms. Instruments come in three main varieties: short period, long period and broadband.
The short and long period measure velocity and are very sensitive, however they 'clip' the signal or go off-scale for ground motion that is strong enough to be felt by people. A bit analog-to-digital conversion channel is commonplace. Practical devices are linear to roughly one part per million. Delivered seismometers come with two styles of output: analog and digital. Analog seismographs require analog recording equipment, possibly including an analog-to-digital converter. The output of a digital seismograph can be simply input to a computer.
It presents the data in a standard digital format often "SE2" over Ethernet. The modern broadband seismograph can record a very broad range of frequencies. It consists of a small "proof mass", confined by electrical forces, driven by sophisticated electronics. As the earth moves, the electronics attempt to hold the mass steady through a feedback circuit. The amount of force necessary to achieve this is then recorded.
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In most designs the electronics holds a mass motionless relative to the frame. This device is called a "force balance accelerometer". It measures acceleration instead of velocity of ground movement. Basically, the distance between the mass and some part of the frame is measured very precisely, by a linear variable differential transformer. Some instruments use a linear variable differential capacitor.
That measurement is then amplified by electronic amplifiers attached to parts of an electronic negative feedback loop. One of the amplified currents from the negative feedback loop drives a coil very like a loudspeaker , except that the coil is attached to the mass, and the magnet is mounted on the frame.
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The result is that the mass stays nearly motionless. Most instruments measure directly the ground motion using the distance sensor. The voltage generated in a sense coil on the mass by the magnet directly measures the instantaneous velocity of the ground. One of the continuing problems with sensitive vertical seismographs is the buoyancy of their masses. The uneven changes in pressure caused by wind blowing on an open window can easily change the density of the air in a room enough to cause a vertical seismograph to show spurious signals.
Therefore, most professional seismographs are sealed in rigid gas-tight enclosures.
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For example, this is why a common Streckeisen model has a thick glass base that must be glued to its pier without bubbles in the glue. It might seem logical to make the heavy magnet serve as a mass, but that subjects the seismograph to errors when the Earth's magnetic field moves.
This is also why seismograph's moving parts are constructed from a material that interacts minimally with magnetic fields. A seismograph is also sensitive to changes in temperature so many instruments are constructed from low expansion materials such as nonmagnetic invar. The hinges on a seismograph are usually patented, and by the time the patent has expired, the design has been improved. The most successful public domain designs use thin foil hinges in a clamp. While there are a number of different BSA equations, the simplest and most commonly used in the hospital setting is a simplified equation by Mosteller Show AMA citation.
Press 'Calculate' to view calculation results. Load an Example. Devine BJ.
Gentamicin therapy. Drug Intell Clin Pharm. The origin of the "ideal" body weight equations. Ann Pharmacother. PMID Experience with a once-daily aminoglycoside program administered to 2, adult patients.
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Antimicrob Agents Chemother. Krenitsky J. Adjusted body weight, pro: evidence to support the use of adjusted body weight in calculating calorie requirements. Weight is usually one of the first topics that come up in discussions about personal health. But the number you see when you step on the scale doesn't tell the whole story about how healthy or unhealthy you may be. Those looking for a clearer picture of what their weight really means in relation to their health may want to take a look at metrics other than weight.
One of these metrics is the body mass index, or BMI, an estimation of body fat that can be calculated using a person's height and weight. Health-conscious individuals might also want to know more about their body composition, which refers to the proportion of fat tissue you have, relative to lean tissue muscles, organs, etc. And anyone can use readily available tools — such as online BMI calculators, skinfold calipers and bioelectrical impedance devices — to better assess their health and physical fitness from the comfort of their own home.
To calculate BMI, divide your weight in pounds by your height in inches squared, and then multiply that number by a conversion factor of For most individuals, a BMI between