Page:The New International Encyclopædia 1st ed. v. 05.djvu/62

* CLOCK. weigliing 4 cwt., which is lifted 9 inches ver- tically from the bell before it falls. This bell and the great bell of Saint Paul's Cathedral are tolled on the death of members of the royal family of Great Britain. In the 'Westmin- ster clock,' as in all the tower-clocks formerly constructed, the mechanism which drives the clock is located in the tower directly back of the face. But in the city-hall clock erected in Philadelphia in 1899 the clockwork is located in the main part of the building, and is connected with the dial-meehanism in the tower by means of compressed air. The whole is based on the fundamental principle of all modern mechanisms - — the governing of great forces by compara- tively feeble ones. The ])rimary clock is an as- tronomical clock, constructed with the greatest care, so as to be free from all disturbances from dust, moisture, and vibrations, and is so con- structed that without interfering with its deli- cacy and accuracy it can operate four sets of hands, each weighing 500 pounds. The dials are the largest in the world, having a diameter of 25 feet. The total height of the tower is 547% feet, and the centres of the dials are 302 feet above the pavement. The hour-hand is 12 feet long, and is entirely different in shape from the minute-liand. so they never can be mistaken. The face of the dial is made up of several pieces, and the usual numerals are omitted from the face, not being of service at so great a height. The clock was designed by Warren S. .Jolnison; a detailed description of its mechanism was written by him, and published in the Journal of the Franklin Institute (Philadelphia) for February. 1901. Electric Clocks. An electric clock is one whose mechanism is, in some way, either actu- ated or controlled by electricity. There are two types of electric clocks — ( 1 ) Independent clocks, whose mechanism is kept in operation by elec- tricity; (2) systems of clocks Which are con- nected with a central or primary clock by an electric circuit, and are so arranged that either (a) the primary clock regulates its movements, at stated intervals, by forcibly moving the hands by an electric current into the proper position, or (b) the primaiy clock directly runs the hands of the secondary clocks, which are simply dials without independent machinery. Clocks of the first class usually have some electromagnetic attachment applied to the mech- anism, which keeps them constantly wound up; in other words, they are constructed like ordi- nary clocks, exce])t that they are self-winding. The first self-winding clocks were made as early as 1855 by Alexander Bain, who applied electro- magnetic attachments to the bob of the pendu- lum, which, in obedience to contacts connected with the pendulum itself, attracted it to and fro. .Since that time many other means of applying the electric current to keep a clock running have been devised, and hundreds of patents for the same have been taken out in England and America ; but, as yet, this form of electric clock has proved of little practical value. An- other form of independent electric clock was put in operation in 189G, in the laboratory of Durham College, Xorth Carolina. The clock is run directly by electroniagTiets. which are actu- ated by a current from an earth battery. The penduhnn is swung by the electric current, and it is this which moves the clock. The works 46 CLOCK and dial are placed on the pendulum and swing with it. In the second class of electric clock the object is to keep a system of clocks correct by an automatic connection with a central and standard timekeeper. It is probaljle that this method of time-service will be rapidly extended, until not only public buildings, but private offices are supplied with time from a central clock, as they now receive a common service in heat, light, power, water, etc. Instead of employing the central clock simply as a regula- tor, it may be adopted as an actual propeller of the clocks on its circuit, and the secondary clocks become mere dials for indicating time. Fig. 6. astronomic.il clock is Philadelphia citv baiAm Certain difficulties have been encountered in the electric connection between the main and sec- ondary clocks, due, among other things, to in- duced currents from crossed telephone or other electric wires. In view of these difficulties, it has been proposed by prominent electricians that the Marconi wireless system be applied to clocks, and thus obviate the faults of the old system. Though the general adoption of elec- trically controlled clocks is a development of the close of the nineteenth century, they have long been the subject of experimentation. In 1837 Alexander Bain began to experiment on electricallv driven s^Tichronous clocks, and in