In its simplest form, a quarter-circle of metal, ivory or wood, with a pair of sights along its upper edge, a scale of degrees inside its curved edge, and a light plumb-bob on a string hanging from its right-angled corner. It is basically an instrument for measuring altitude, but different models have been developed for a variety of purposes.
A quadrant intended solely or almost solely for finding the time is termed a 'horary quadrant'; it is in effect an altitude sundial. Horary quadrants have been made to indicate unequal or equal hours, and sometimes both. There are two methods of arranging for a date setting. In one, the date scale is marked vertically near one straight edge of the quadrant, and a slider on the plumb-bob line is set to this. Then a sight of the sun is taken and the string held against the quadrant face with the thumb, the slider giving the reading on the curved hour lines. In the other method, the date scale is near the curved edge of the quadrant, the line is set to the date and its slider moved to the point where it meets the 12 line, the same subsequent procedure being followed. A fine horary quadrant indicating unequal hours, dating from the early 14th century, is in Merton College, Oxford. The earliest surviving dated quadrants showing equal hours are one of 1398 in Dorchester Museum and a similar example of 1399 in the British Museum.
A development of the simple horary quadrant was the so-called 'Gunter' quadrant, invented c. 1618 by Edmund Gunter of Christ Church, Oxford. This is engraved with two sets of lines side by side on the face of the quadrant. The set to the left forms a horary quadrant while that to the right enables the sun's azimuth to be found when its altitude is known at a given date. An unusual feature is that both sets of lines are folded over at the equinox line to permit a more open scale, but giving a complex appearance of crossed lines.
Other special forms of quadrant are the sinical quadrant, whose scales enable the sines of the measured angles to be read off directly, and gunnery quadrants, from which the angle of elevation of the target and the necessary elevation of the gun barrel can be determined. Very large fixed quadrants for accurate measurement of the altitude of stars were essential pieces of equipment in pioneer astronomical observatories.
A crystal of quartz suitably cut and mounted will produce an electrical effect when mechanically deformed; conversely, it mechanically deforms when subjected to an electrical field. This is known as the 'piezo-electric effect'. When a suitably cut quartz crystal is connected in an electrical circuit, it can be made to vibrate at its natural frequency, and may be used to control the frequency of an electronic oscillator which, after suitable frequency division, will indicate time on a digital or analogue display.
Shortly after the excellent stability of frequency control by quartz crystal when applied to valve oscillator circuits was discovered, efforts were directed to producing a clock using this principle for time measurement. Warren A. Marrison of New York was pre-eminent in the application of quartz-crystal clocks to timekeeping, producing his first design c. 1929. Progress in the understanding of the quartz-crystal oscillator and the associated circuits needed to divide the high frequency down to a suitable value for operating small synchronous motors let to the quartz-crystal clock being adopted as a time-measurement standard in place of the pendulum clock in astronomical observatories from c. 1943. A frequency of 100,000 hertz was adopted in all the early clocks, the quartz crystal being kept at a constant temperature by an electrically heated oven. Dr. Louis Essen of the National Physical Laboratory developed the quartz-crystal ring, greatly improving the stability of frequency to about one part in one hundred million, and these were adopted as NPL standards for frequency and time measurement; this represents about 0.001 second in 24 hours. Modern quartz-crystal oscillators for clocks often operate at 32,768 hertz, which is divided down by binary circuits to 1 hertz for the direct operation of the seconds hand of the clock in the case of analogue display.
A device by which a spindle may be rotated and at the same time moved axially, as in the driving spindle of a bench drill in which the drill can travel to and from the work while spinning. It may take the form of a sleeve which does not itself rotate, but may be moved axially, usually having its outer surface machined with rack teeth to engage with the teeth of the feeding mechanism. The spindle is carried in bearings fixed inside the quill, and its driving end is provided with a long groove or keyway which engages with a corresponding key inside the bore of the pulley which rotates it.