The adjusting pin should be well fitted to the holes in the front iron bar, and also fit the corresponding ones in the zinc bar closely, and if the holes are reamed smooth and true with an English clock broach, then the pin will be slightly tapering and fit the iron hole perfectly solid. After one pair of these holes have been reamed, fit the pin and drive it in place perfectly firm, and then with the broach ream all the remaining holes to just the same diameter, and then the pin will move along from one set of holes to another with mechanically accurate results. Otherwise, if poorly fitted, the full effect would not be obtained from the compensating action in making changes in the pin from one set of holes to another. This pin, if made of cast steel, hardened and drawn to a blue, will on the whole be a very good device mechanically.
Many means are used to effect the adjustments for compensation, of more or less value, but whatever the means used, it must be kept in mind that extra care must be taken to have the mechanical execution first class, as on this very much depends the steady rate of the pendulum in after time.
Tubular Compensated Rods.—There are tubular pendulums in the market which have a screw sleeve at the top of the zinc element, and by this means the adjustments are effected, and this is thought to be a very accurate mechanism. The most common form of zinc and iron compensation is where the zinc is a tube combined with one iron tube and a central rod, as shown in [Figs. 10, 11, 12]. The rod is the center piece, the zinc tube next, followed by the iron tube enveloping both. The relative lengths may be the same as those just given in the foregoing example with the compensating elements flat. The relative lengths of the several members will be virtually the same in both combinations.
Tubular Compensation with Aluminum.—The pendulum as seen by an observer appears to him as being a simple single rod pendulum. [Figs. 10] and [12] are front and side views; [Fig. 11] is an enlarged view of its parts, the upper being a sectional view. Its principal features are: The steel rod S, [Fig. 11], 4 mm. in diameter, having at its upper end a hook for fastening to the suspension spring in the usual way; the lower end has a pivot carrying the bushing, T, which solidly connects the steel rod, S, with the aluminum tube, A, the latter being 10 mm. in diameter and its sides 1.5 mm. in thickness of the wall.
The upper end of the aluminum tube is very close to the pendulum hook and is also provided with a bushing, P, [Fig. 11]. This bushing is permanently connected at the upper end of the aluminum tube with a steel tube, R, 16 mm. in diameter and 1 mm. in thickness. The outer steel tube is the only one that is visible and it supports the bob, the lower part being furnished with a fine thread on which the regulating nut, O, is movable, at the center of the bob.
For securing a central alignment of the steel rod, S, at its lowest part, where it is pivoted, a bushing, M, [Fig. 11], is screwed into the steel tube, R. The lower end of the steel tube, R, projects considerably below the lenticular bob ([compare Figs. 10 and 12]); and is also provided with a thread and regulating weight, G ([Figs. 10 and 12)], of 100 grammes in weight, which is only used in the fine regulation of small variations from correct time.
The steel tube is open at the bottom and the index at its lower end is fastened to a bridge. Furthermore all three of the bushings, P, T and M, have each three radial cuts, which will permit the surrounding air to act equally and at the same time on the steel rod, S, the aluminum tube. A, and the steel tube, R, and as the steel tube, R, is open at its lower end, and as there is also a certain amount of space between the tubes, the steel rod, and the radial openings in the bushings, there will be a draught of air passing through them, which will allow the thin-walled tubes and thin steel rod to promptly and equally adapt themselves to the temperature of the air.
Fig. 10. Fig. 11.Fig. 12.