. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 220 BULLARD [chap. 1 1 or a vibrating reed may be used between the bridge and the ampHfier. A system of relays or a rotary switch is necessary to connect the thermistors successively to the bridge; calibrating resistors may be included in the cycle so that the resistance of the thermistors is obtained by interpolation and not by assumiiig the zero and sensitivity of the system to be known. A check on the resistance of the thermistors when at the same temperature may be obtained by noting the readings when the instrument is
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 220 BULLARD [chap. 1 1 or a vibrating reed may be used between the bridge and the ampHfier. A system of relays or a rotary switch is necessary to connect the thermistors successively to the bridge; calibrating resistors may be included in the cycle so that the resistance of the thermistors is obtained by interpolation and not by assumiiig the zero and sensitivity of the system to be known. A check on the resistance of the thermistors when at the same temperature may be obtained by noting the readings when the instrument is suspended a short distance above the bottom. When a probe weighing 160 kg in air and 130 kg in water strikes the bottom at a speed of 3 m/sec and penetrates 5 m, 1700 cal of heat are produced. This heat appears partly in the probe and partly in the sediment; if it all appeared. 20 Time (min) Fig. 2. Variation with time of the temperature difference between the top and the bottom of a probe cm in diameter. The full-line curve represents the theoretical ex- pression (1). (After Bullard et al., 1956, fig. 2.) in the probe used in Discovery II it would raise its temperature by °C, which is considerably larger than the temi^erature difference to be measured. In practice part of the heat goes into the sediment. The time taken for heat to spread from the surface of the probe to the centre is only a few minutes and is much shorter than the time needed for the probe to reach temperature equilibrium with the sediment. If the lower end of the probe is heated above the temperature of the sediment into which it has penetrated the temperature will rise for about 3 min, pass through a maximum and then fall towards its final value as shown in Fig. 2. It is not practicable to leave a probe in the bottom for more than about 40 min; after this time the temperature gradient in the probe usually exceeds that in the sediment by 10 to 15%. It is, therefore, desirable to extrapolate the obser
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