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London Crystal: And at the Great Exhibition of the Works of Industry, held in rhe Crystal Palace in london Crystal in 1851, Americans won three of the five medals awarded for daguerreotypes. Horace Greeley, editor of the New %rk Tribune, wrote from london Crystal: "In Daguerreotypes it seems to be conceded that we beat the world,, where excellence and cheapness is both considered—^ all events, England is no where in comparison—and out}
The second law has a more fundamental interpretation. For example, suppose a small crystal initially at room temperature is placed inside an oven that is maintained at a constant high temperature. The crystal is a highly ordered structure not in thermal equilibrium with the oven walls. By the second law of thermodynamics heat flows from the hot walls into the crystal, not in the reverse direction.
In order to ensure close-frequency control a quartz crystal is substituted for the L-C circuit, and usually some other modifications also are made. A quartz crystal provides a much sharper (more narrow width) response curve and thus holds the operating frequency closer to the desired frequency than does the simple L-C tuned circuit.
The frequency at which a quartz crystal operates is determined by its dimensions, particularly its thickness. For operation at frequencies greater than about 15 MHz, it must be so thin as to be extremely fragile.
This disadvantage is circumvented in the following way. When a quartz crystal is used to control operating frequencies greater than about 15 MHz, the oscillator operates within the lower frequency range of the quartz crystal, and then the oscillator output signal is fed to one or a series of frequency multipliers. |
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