It should be understood that under this term two, or we may say three, very different things are comprehended. The first is a mere clock movements, i.e., the works of a clock without either weight or pendulum, which is kept going by electrical connection with some other clock of any kind (these ought to be called electrical dials, not clocks); the second is a clock with a weight, but with the escapement worked by electrical connection with another clock instead of by a pendulum; and the third alone are truly electrical clocks, the motives power being electricity instead of gravity; for although they have a pendulum, which of course swings by the action of gravity, yet the requisite impulse for maintaining its vibrations against friction and resistance of the air is supplied by a galvanic battery, instead of by the winding up of a weight.
If you take the weight off a common recoil escapement clock, and work the pallets backward and forwards by hand, you will drive the hands round, only the wrong way; consequently, if the escapement is reversed, and the pallets are driven by magnets alternatively made and unmade, by the well-known method of sending an electrical current through a wire coil set round a bar of soft iron, the contact being made at every beat of the pendulum of a standard clock, the clock without the weight will evidently keep exact time with the standard clock; and the only question is as to the best mode of making the contact, which is not so easy a matter as it appears to be, and though various plans apparently succeeded for a time, and were mechanically perfect, not one has succeeded permanently; i.e., the contact sometimes fails to produce the current of sufficient strength to lift the weight or spring on which the driving of the subordinate clock depends. It is therefore unnecessary to repeat the description of the various contrivances for this purpose by Wheatstone and others.
The first person who succeeded in making one clock regulate or govern other by electricity, Mr R. L. Jones, accordingly abandoned the idea of electrical driving of one clock by another; and instead of making the electrical connection with a standard clock (whether itself an electrical one or not) drive the others, he makes it simply let the pallets or the pendulum of the subordinate clock, driven by a weight or spring, be influenced by attraction at every beat of the standard clock; and, by way of helping it, the pallets are made what we called half-dead in describing the dead escapement, except that they have no impulse faces, by the dead faces have just so much slope that they would overcome their own friction, and escape of themselves under the pressure of the clock train, except while they are held by the magnet, which is formed at every beat of the standard clock, or at every half-minute contact, if it is intended to work the dials by half-minute jumps. This plan has been extensively used for regulating distant clocks from Greenwich Observatory.
The first electrical clocks, in the proper sense of the term, were invented by Mr Bain in 1840, who availed himself of the discovery of Oersted that a coil of insulated wire in the form of a hollow cylinder is attracted in one direction or the other by a permanent magnet within the coil, not touching it, when the ends of the coil are connected with the poles of a battery ; and if the connection is reversed, or the poles changed, so that the current at one time goes one way through the coil from theor copper plate to the + or zinc plate, and at other times the other way, the direction of the attraction is reversed. Mr Bain made the bob of his pendulum of such a coil enclosed in a brass case so that it looked like a hollow brass cylinder lying horizontal and moving in the direction of its own axis, and in that axis stood the ends of two permanent magnets with the north poles pointed at each other and nearly touching, as in the right hand part of fig. 16. The pendulum pushed a small sliding bar backwards and forwards so as to reverse the current through the coil as the pendulum passed the middle of the arc, and so caused each magnet in turn to attract the bob. But this also failed practically, and especially in time-keeping, as might have been expected, from the friction and varying resistance of the bar to the motion of the pendulum, and in the attractions.
Mr Ritchie of Edinburgh, however, has combined the principle of Bains and Joness clocks in a manner which is testified to be completely successful in enabling one standard clock to control and keep going any number of subordinate ones, which do not require winding up as Joness do, but are driven entirely by their pendulums. This differs from Wheatstones plan in this, that his subordinate clocks had no pendulum swinging naturally and only wanting its vibrations helping a little, but the pallets had to be made to vibrate solely by the electrical force. The figures are taken from Mr Ritchies paper read before the Royal Scottish Society of Arts in 1873. The controlled pendulum P is that just now described as Bains (seen in fig. 17 the other way, across the plane of vibration); the rod and spring are double, and the wire cd is connected with one spring and rod (say the front one) and the wire de with the other; so that the current has to pass down one spring and one rod and through the coil in the bob an dup the other spring. The other pendulum O of the normal or standard clock is a common one, except that it touches two slight contact springs a,b alternately, and so closes the circuit on one side and leaves it broken on the other. When that pendulum touches a the B battery does nothing, and thecurrent from the battery A passes by a to c and d and down the d spring and rod and up through d to e and backagain to + of A. But when the standard pendulum O touches b the A battery does nothing, and the current from to + of the B battery goes the other way, through the controlled pendulum and its coil. The two fixed magnets SN, NS consequently attract the coil and bob each way alternately. And even if the current is occasionally weak, the natural swing of the pendulum will keep it going for a short time with force enough to drive its clock through a reversed escapement; and further, if that pendulum is naturally a little too fast or too slow the attraction from the standard pendulums will retard or accelerate it. In practice, however, it is found better not to make the contact by springs, which, however light, disturb the pendulum a little, but by a wheel in the train making and breaking contact a every beat; and if the clock has a gravity escapement there is no danger of this affecting the pendulum at all.
In order to get the machinery into a smaller compass than a 39 inches pendulum requires, Mr Ritchie uses a short and slow pendulum with two bobs, one above and the other below the suspension, as shown in fig. 17. Such a pendulum, like a common scale-beam, may be made to vibrate as slow as you like by bringing the suspension nearer to the centre of gravity of the whole mass. But they are quite unfit for independent clock pendulums, having very little regulating power, or what we may call force of vibration. He applies magnets to both the bobs, so as to double the electrical force. Fig. 17 is the section across the plane of vibration.
Fig. 18 shows the kind of reversed escapement, or "propelment," sued with these short and slow pendulums. The pendulum here is returning from the extreme right, and has just deposited the right hand pallet BCD with its end D pressing on a tooth of the scape-wheel, but unable to turn it because another tooth is held by the stop G on the left pallet. As soon as the pendulum lifts that pallet the weight of the other pallet turns the wheel, until a tooth falls against the stop C. When the pendulum returns from the left the left pallet presses on a tooth at E but cannot turn the wheel because it is yet held by Cm until that is released . In order to prevent the hands being driven back by wind where they are exposed to it, a click is added to the teeth. The wind cannot drive the hands forward by reason of the stops C, G.
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