ELECTRIC CLOCK. The regularity of the clock depends, as is well known, on the action of the pendulum, which is isochronous—that is, has the property, within cer tain limits, of describing long and short arcs in the same time. See PENDULUM. The pendulum, however, left to itself, would, in consequence of the resistance of the air, and of the spring on which it hangs, soon cow.; to rest. An impulse must therefore be given it occasionally, to keep it going. This impulse need not necessarily be exactly the same, for though it might cause the pendulum to make a longer swing at one time than at another, the time of oscillation would not differ. In ordinary clocks, these impulses are given by a heavy weight, and are transmitted to the pendulum through the wheel-work of the clock. No moving power can be more steady than gravity, or less likely to tax the isochronism of the pendulum, but its action on ilie clock is limited by the distance through which the weight descends, so that the weight must be periodi cally wound up, to keep gravity in play. The trouble of winding, though small, still leaves room for the wish that a clock might be constructed going for long periods with out external help. Such an instrument the E. C. professes to be; but an independent E. C. is not trustworthy as a timekeeper, and all that electric clocks are used for is to copy the time of a good gravity clock. This work the E. C. does to perfection. The E. C. was invented by Bain, an Edinburgh clock maker, in 1840, and his ideas, though improved and modified, still form the basis of electric clock-making. In the ordinary clock, it is the clock that moves the pendulum; in Bain's clock, it is the pendulum that moves the clock. As the construction of the pendulum is the only part of it connected with electricity, we shall confine our notice to a general description of the pendulum action. To the lower part of the pendulum attached; congaing of a hollow bobbin of insulated copper wire. Wires from both ends ascend the pendulum-rod, and are in metallic connection respectively with the two springs from which the pendulum hangs. Two magnets, or bundles of magnetic rods, are fixed at either side of the bob, and are of such dimensions that the hollow bob in its oscillation can pass a certain way over each without touching. The magnets have their like—e.g.—south, poles turned towards each other. The two springs of the pendulum-rod are in connection with the two poles of a galvanic battery. The wire connecting one of them is made to pass round by a break, worked by the pendulum-rod. When the pendulum is made to move, say towards the right, it shifts a slider, so as to complete the connection between the • poles of the battery. The current thereupon descends one of the wires of the pendulum,
passes through the coil of wire forming the bob, and ascends by the other. In so doing, it converts the bob into a temporary magnet, the s. pole towards the right, and the n. pole towards the left. In this way, the s. pole of the bob is repelled by the s. pole of the right-hand magnet; and its n. pole is attracted by the s. pole of the left-hand magnet, so that from this double repulsion and attraction, both acting in the same direction, the bob receives an impulse towards the left. Partly, therefore, from this impulse, and partly from its own weight, the pendulum describes its left oscillation; and when it reaches the end of it, it moves the slider so as to cut off the battery cur rent, and then returns towards the right, under the action simply of its own weight. On reaching the extreme right, as before, it receives a fresh impulse; and thus, under the electric force exerted during its left oscillation, the motion of the pendulum is maintained. So long as the electricity is supplied, will the pendulum continue to move. The current required is exceedingly weak, and Bain considered that it could be sufficiently excited by a plate of copper and a plate of zinc sunk into the ground, and acted upon by the moisture usually found there. This earth-battery, as lie called it, was expected to act steadily for but the result proved far otherwise, for the soil not unfrequently dried up, leaving no trace of electrical action. The imperfection of the battery has led to a strong prejudice against these clocks—stronger, certainly, than they merit. Practice has, however, established that a clock driven by an electric pen dulum, under no control, is not to be trusted, and clocks of this kind, so far at least as this country is concerned, are entirely abandoned. The next important step in perfect ing the E. C. was made by Lewis Jones (patented 1857). All his clocks are ordinary gravity clocks. The standard clock is not an E. C. at all, but its pendulum makes and breaks contact in the battery circuit which controls the copying clocks. These last, though driven by weights, have Bain's pendulums, and the currents transmitted by the standard clock keep them oscillating in exact accordance with it, so that the standard clock and copying clocks have their pendulums always at exactly the same point in their oscillations. The copying clocks are adjusted to keep nearly the time of the pri mary, and the margin of error is wholly removed by electric control.