Clock

(klok.) [From Fr. cloche, a bell.] (Horol.) A machine for measuring time. Its invention has been attributed to Pacific us, archdeacon of Verona, in the 9th cent.; and even to Bœthius, in the early part of the 6th. The most ancient C., of which we have any distinct account, were that erected in a tower of the palace of Charles V. of France, about the year 1364, and that made at Strasburg about the year 1370. In the following cent. public C. became very common in Europe. The first method used for regulating C. was by a flywheel; but this, being affected by the variable resistance of the air, acted very imperfectly. The great improvement in clock making was the application of the pendulum in the 17th cent.; but it is uncertain by whom itwas first employed. Some attribute it to Galileo, who first announced its isochronism; and others to Huygens, who first explained its principle. The compensation pendulum was invented by Hooke in 1715; and the method of compensation by the unequal expansion of different metals by Graham. A C. consists of wheels moved by weights, and is so constructed that, by a uniform vibration of a pendulum, the hours, minutes, and seconds are measured with great exactne ; and it indicates the hour, not only by a dial, but by the stroke of a hammer on a bell. Sometimes, also, it strikes the half-hours and quarters. The C. measures 24 mean hours, but the solar day is of various lengths, according to the situation of the earth in its orbit, and to the declination of the sun. Hence the C. is sometimes a fow minutes faster or slower than the sun. Electrical C. is a clockwork machine, in which an electro-magnet, by means of an electric current regularly interrupted, is both the motor and the regulator. Fig. 189 represents the mechanism of such a C., which works the needles. An electro-magnet, B, attracts an P Fig. 189.-ELECTRICAL CLOCK. armature of soft iron, P, movable on a pivot, a. The armature P transmits its oscillating motion to a lover, 8, which, by means of a ratchet, n, turns the wheel A. This, by the pinion D, turns the wheel C, which, by a series of wheels and pinions, moves the hands. The small one marks the hours, the large one the minutes; but as the latter does not move regularly, but by sudden starts from second to second, it follows that it may also be used to indicate the seconds. It is obvious that the regularity of the motion of the hands depends on the regularity of the oscillations of the pi eco P. For this purpose, the oscillations of the current, before pa ing into the electro-magnet B, are regulated by a standard C., which itself has been previously regulated by a seconds pendulum. At each oscillation of the pendulum the current is opened and closed, and thus the armature P beats the seconds exactly. To illustrate the use of these electrical C., suppose that on the railroad from New York to San Francisco, each station has an electric C., and that from the New York station a conducting wire pa es to all the C. on the line as far as San Francisco. When the current pa es in this wire, all the C. will simultaneously indicate the same hour, the same minute, and the same second; for electricity travels with such prodigious velocity, that it takes an in appreciable time to go from New York to San Francisco. [s. 198]