Figure 4 A Schematic of a Geiger tube with three avalanches initiated from a

Figure 4 a schematic of a geiger tube with three

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Figure 4: A Schematic of a Geiger tube with three avalanches initiated from a single ionised electron.
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4–1 Laboratory exercise 4 Laboratory Exercise 4 – DIGITAL THERMOMETRY There are two parts to this exercise, which takes two 3-hour laboratory sessions. In part A you make and calibrate a direct-reading digital thermometer usable over the range 0–100 °C. In part B you use this thermometer to investigate the rate at which various liquids cool. There are few instructions provided for part B — instead you are given a copy of a published study of this topic, and asked to repeat the procedures and check the conclusions for yourself. You must write a short formal report on this exercise, using the published paper as a model. S ubmit only the report . There is more on what must be included in the report on page 4–4. Because the calibration of the thermometer is very sensitive to any changes in its electrical circuit, you should do parts A and B on successive days , Monday/Tuesday or Thursday/Friday. Your circuit will be left undisturbed on the bench between the two sessions. Making the thermometer requires you to use and understand some of the electrical circuits covered in experiment 1, which you must have completed before starting this exercise. Part A: Making the digital thermometer Introduction The temperature sensor is a semiconductor diode whose resistance varies with temperature. The diode is used as one arm of a Wheatstone bridge circuit, which will therefore only balance at one temperature. The off-balance voltage is measured with a digital multimeter (DMM) and adjusted with a potential divider to give a direct digital reading, in mV, of the temperature in degrees Celsius. There are three separate tasks: (i) measure the properties of the diode related to its temperature dependence; (ii) set up and adjust the Wheatstone bridge circuit; (iii) use a potential divider to adjust, calibrate and check the performance of your thermometer. You should allocate at most an hour to each of these tasks (including tabulating and plotting data), so as to complete part A in the first afternoon. Characteristics of the diode Diodes are electrical devices which allow current to pass in only one direction, the forward direction. In the reverse direction they have a high resistance, and so can act as one-way switches. The symbol for a diode is with the arrowhead indicating the forward direction, so a voltage applied thus causes current to flow; the diode is then said to be forward biased . To make a diode, a semiconducting material (silicon in this case) is doped with an impurity in order to give a deficit of electrons, hence excess p ositive charge, in one region, and another impurity to give an excess of electrons, hence excess n egative charge, in an adjacent region. The boundary between the regions is the junction . So the diodes you use here are called silicon p–n junction diodes . The diodes themselves are small, the size of a match head; the one you use has been encased in insulating mastic with only its two electrical leads left exposed.
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  • Summer '09
  • Electrical resistance, Electrical network, Voltage drop, Thermometer, Geiger tube

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