1851 Great Exhibition: Official Catalogue: Class X.: Charles Shepherd
128. SHEPHERD, CHARLES, 53 Leadenhall Street — Inventor and Patentee.
Patent electro-magnetic striking clock. From the pendulum of this clock, a number of dials may be worked. The greatest novelty consists in the method of giving the impulse by means of a remontoir escapement, by which the variations of the battery take no effect on the time measured. The novelty of the Large Clock in the Transept of the Exhibition Building, in connexion with the former, is in the method of locking the escape wheel, to prevent the train from running by the action of the wind on the hands.
Two dials, five feet in diameter each.
A skeleton electro-magnetic striking clock, showing how the number of blows to be struck is regulated.
Small turret bell, illustrating the method of applying electro-magnetism to move the hammer.
The power employed for keeping in continual action the electric clock, is one of Smee's batteries in connexion with a powerful horse-shoe magnet. In the case of the mechanism of the great clock now under consideration, a series of such magnets is used, the connexion of which, with their armature, is shown in fig. 1. These are surrounded by 25,000 feet, or nearly five miles of No. 18 copper wire, the total weight of which is about 160 lbs. As weights are entirely dispensed with, the frame containing the wheel-work is much lighter than usual; the escape-wheel, a, a, fig. 2, is 10 inches in diameter, and is in two parts, the teeth of each being reversed; the click and ratchet escapement, which is moved by the electro-magnets, acts on the teeth of one of the parts, while the teeth of the other part are used for the purpose of locking the train, in order to prevent it running forward from the action of the wind on the extra-sized hands which present a large surface. A central vertical wheel, b, of larger diameter (see fig. 2 above, and fig. 3, opposite page), which works into a pinion, c, on the arbor of the escape-wheel, gives motion to the wheel-work in connexion with the hands, which are at a height of 40 feet above the pedestal in the South Gallery of the Transept on which the machinery is placed, the communication being effected by a 2-inch bevelled wheel, d, which rotates on the end of the spindle of the great wheel, and works into a horizontal bevelled wheel, e, with which a vertical shaft, f, made of brass tubes of 1 inch diameter, and 5 screwed together in several lengths, revolves, and which, in connexion with wheel-work at top, gives motion to the hands.
In order that the clock-face might harmonize with the design of the south elevation of the Transept, it was considered that the conventional form of a circle for the dial might be dispensed with; the figures were accordingly arranged in a semicircle, and placed at the intersections of the radial bars with the second semi-ring from the centre of the great fanlight.—(See fig. 4, opposite page.)
As is the case in ordinary dials, so in the present instance, XII. is at the top, I. to VI. following on the right hand side of the semicircle; while on the left side the figure VI. is repeated, and the remaining figures up to XI. inclusive follow in the usual order. In order to render the new form of dial perfectly useful, it was necessary to have two minute hands, and also two hour hands; so that when one of the minute hands, for instance, leaves the figure VI. on the right hand, the other minute hand also points to the corresponding VI. on the left hand. The two minute hands together are 16 feet, and the two hour hands 12 feet in length respectively. Two smaller dials, each of five feet diameter, are fixed up inside the building; one in front of the cross gallery at the east end of the central aisle, and the other in front of the south gallery of the transept. All the dials are regulated by one pendulum, represented in fig. 5, and which is suspended from a triangular frame, g, resting on a bed-plate, to which it is secured by screw-bolts in the ordinary way. The pendulum has a mercurial compensation for heat and cold, and is kept in motion by electro-magnetism by a method entirely different to any previously invented. Instead of applying directly the
attractive and repulsive forces of electro-magnetism to the pendulum, according to all the previous methods, the power of an electro-magnet is here employed to bend a
spring to a certain fixed extent, the reaction of which gives the necessary impulse to the pendulum, by which means the variations which are continually taking place in the batteries have no effect on the time measured. The arrangement of the spring for giving the impulse is represented at s (fig. 5), in which b is the impulse spring, consisting of a short steel spring, to which are attached two arms, c and d, at right angles to each other; e is the detent, or catch, for holding this spring when bent by the action of the magnet; f is the pendulum-rod carrying the two screws, h and i, which may be called the impulse and discharging pallets. As the pendulum vibrates to the left, the discharging-pallet pressing against the perpendicular arm of the detent, c, forces it into the position indicated by the curved lines; the impulse spring is thus liberated and immediately falls against the impulse pallet h. As the pendulum returns to the right, the impulse spring by its elasticity urges the pendulum forward with the exact power required to continue its vibration. The spring is limited in its motion by a screw, o, by screwing or unscrewing which the length of the stroke of the spring, and consequently the power may be regulated to the greatest nicety. The pendulum, continuing its motion to the right, comes in contact with a slight spring tipped with platina, which completes the circuit of the galvanic battery through the coils of the electro-magnet, which is placed immediately underneath the bed-plate in an inverted position, the poles of which pass through the bed-plate. An armature, k, con, sisting of a flat bar of iron, is placed immediately above the poles, being attached to a horizontal arm at right angles thereto, which arm moves freely on an axis x, properly supported at either end on a bracket. On the opposite side of the axis is another arm also projecting at right angles, but considerably longer than the first. The use of the second arm, in connexion with an adjustable weight, is to raise the armature from the poles of the magnet. When, therefore, a current of electricity is made to pass through the coils of wire which surround the magnet, the armature is attracted towards the poles, and consequently the long arm with the adjustable weight is elevated in the opposite direction. It is evident that this arm cannot be raised without lifting the arm, d, of the impulse spring, b, bending the impulse spring, and locking the upper end of the arm, c, on the detent, ready for the next impulse.
[A point of importance in the construction of this clock is the method of making and breaking contact for the electric currents. When the circuit is broken, a spark is seen to pass between the points of contact. The continued action of this spark causes the points between which it passes, to become oxidised; and as the
metallic oxides are non-conductors of electricity, it follows that the passage of the electricity will be thereby interfered with and prevented.
In the first clock constructed, a piece of steel-wire was used as a break spring, touching against the side of the pendulum-rod; but the points of contact oxidised so rapidly, that the clock would not go for more than a few days without stopping. The steel spring vas then removed, and one of gold substituted, and a- small plate of gold was soldered to the side of the pendulum-rod. The difficulty appeared to have been entirely overcome; but in six weeks the quantity of electricity passing was considerably reduced, and at the end of two months the clock stopped.
Platina was next tried, in the same manner as the gold, in a new clock, completed in 1848, since which time the points of contact have never required cleaning, the circuits being completed at the present time with as much certainty as when the clock was first put together. These points of contact present some peculiarities when examined with a lens. With metals having a great affinity for oxygen, a black spot forms immediately at the point of contact; while in the case of platina the immediate point of contact remains perfectly clean, a rim of black forming around it. This black rim has been found to be quite capable of conducting electricity. The probable conclusion, therefore, is, that the black rim is platina in a very fine state of division, and not an oxide of the metal.
The battery best adapted for these clocks is that of Mr. Smee, both on account of its simplicity and the ease with which it is recharged. The amalgamated zinc employed in this battery is subject to rapid local action, by the quantity of impurity which it contains, consisting usually of lead, iron, copper, tin, and cadmium; all these metals having less affinity for oxygen than zinc, become negative when immersed in dilute acid, and form a voltaic circle with the surrounding particles of zinc. The use of amalgamation is to stop this action, which, when the amalgamation is fresh, it accomplishes; but in a few days the local action again commences, and increases until the acid is neutralised, or the whole of the zinc dissolved.
This may be obviated by standing the zinc plate loose in the jar, resting on the bottom, and pouring in an ounce or two of mercury. This plan is found to answer remarkably well; the quicksilver soaking up the zinc plate, keeps it thoroughly amalgamated. The zinc may be melted, and after being mixed with mercury, cast in moulds; the quicksilver would then form one of the impurities present: and should local action take place at any one point, the solution of the zinc would not only liberate the other metals present, but liberate at the same time sufficient quicksilver to cover them, and stop the local action.
It is well known that the zinc of a battery is acted upon more severely at the surface of the liquid than elsewhere, by which the lower part is wasted. This appears to depend on the presence of oxygen; for it does not go on, where the battery has been enclosed in a bottle to collect the hydrogen evolved. A double advantage results from making the batteries air-tight; not only is this peculiar action stopped, but the evaporation of the water prevented. In batteries required to act for long periods, one zinc plate should be employed, as when two are used, one of them almost always becomes negative to the other; and although this action is very slight, yet when it continues constantly for several months, its effect is very perceptible.
This clock, although quite equal to that of St. Paul's Cathedral, occupies far less space; the heavy weights, with the room necessary for their descent, being of course dispensed with. One of the most obvious advantages in electro-magnetic clocks is, that precisely similar time will be kept by any number of dials situated in the different parts of a large establishment, and connected with one pendulum. Such a series has been in operation for some time at an extensive commercial warehouse. The whole of the dials are regulated by one pendulum, situated in the counting-house. The wire required to communicate between the pendulum and the dials in the different departments of the warehouse is upwards of a quarter of a mile in length.]